Best Practice Guidelines for Standalone PV Systems

Standalone PV Systems

Best Practice Guideline THIS PAGE TO HAVE ACKNOWLEDGEMENTS ETC 3 Glossary a.c. bus system A PV system r stand alone or hybrid) where the PV array is connected to the d.c side of the system, that is batteries via a controller Alternating Current Is electricity in which the polarity of the current is periodically reversed. (AC) Altitude Is the height of the Sun from the horizon. Ampere-hours The unit used to measure electrical energy and to indicate the storage capacity in batteries. Symbol, Ah. To convert Ah to kWh multiply the Ah by the voltage then divide by 1000. Azimuth Is the east–west position of the Sun. The solar industry standard is to express azimuth clockwise from true north (0°–360°); however it can also be quoted with a direction, east or west (i.e. 0°–180°E or 0°–180°W). Battery Electrical energy storage device. Two or more electrochemical cells electrically connected together in a series or parallel combination to provide the required operating voltage and current levels. Battery Capacity The maximum total electrical charge, expressed in ampere-hours (Ah) that a battery can deliver to a load under a specific set of conditions. The battery capacity depends on the rate at which the charge is given up by the battery. Battery Charging The voltage specified as that required to charge batteries. For 12V batteries the final Voltage battery charging voltage is around 14V and photovoltaic modules are often rated at 14V so that the charging current is known at that voltage. Charging Rate The current applied to a battery or cell to restore its capacity. The charging rate is specified by the CX where x is the number of hours to bring the battery to full charge, eg. if a battery with capacity 100 Ah is charged at the C5 rate then it is brought to full charge in 5 hours and 20 Amperes would be the charging current. Central inverter Usually a collection of small inverters centrally located in one housing. Circuit A circuit is the path that current flows from one charged point to another. Circuit breaker A mechanical device which will open a circuit under fault conditions. When too much current passes through, the device will open and prevent current flow. The circuit breaker can then be manually operated to close the circuit. Combiner box An electrical component for combining and housing the wiring from the PV array. Current The rate of flow of electrical charge is the net transfer of electrical charge per unit time. The unit of current is the Ampere (A). In electric circuits the current is referred to by the symbol (I). Daily Demand or This is the energy requirement calculated on a daily basis. This load varies from day Daily Load to day and at different times of the year. In sizing a system the average daily demand over the whole of the year is often used. Units can be Wh, kWh, or Ah. Days of Autonomy The number of days, without input from the energy source, for which the battery storage system must supply electrical energy. d.c. bus system A PV system (stand alone or hybrid) where the PV array is connected to the d.c side of the system, that is batteries via a controller Decisive voltage Is a classification system for different voltage ranges. classification (DVC) Depth of Discharge The Ahs removed from a cell or battery expressed as a percentage of the rated (DOD) capacity, eg., the removal of 25 Ah from a fully charged 100 Ah rated battery results in a 25% depth of discharge. Direct Current (DC) Is electricity in which the current always moves in the same direction. Discharge Rate The current removed from a cell or battery. Battery manufacturers refer to the rate

4 of discharge not by Amps but by the time it would take to completely discharge the battery. For example a battery which has a rated capacity of 100 Ah is subjected to a current withdrawal of 5 A then it would take 20 hours to completely discharge the battery. In this case we say that the battery is discharging at the 20 hour rate or at C20. Disconnector Is a mechanical switching device which provides, in the open position, an isolating distance in accordance with specified requirements. Efficiency The ratio of energy (power) produced by a device to the energy (power) consumed by the same device. It is a number less than or equal to unity. Electrolyte A non-metallic electric conductor in which current is carried by the movement of ions. In lead acid batteries the electrolyte is an aqueous solution of sulphuric acid. Energy Is the amount of electric energy transferred, a product of power and time. Energy is measured in watt hours (Wh) and is calculated using: E = P x t Equalisation The process of restoring all cells in a battery to an equal state of charge. IN a lead acid battery this is done by using a charging voltage of around 2.5 Volts per cell. Equipotential Equipotential bonding (or protective earthing) involves electrically connecting bonding earthed, conductive metalwork so that it is at the same voltage (potential) as earth throughout. This is required for safety reasons to protect people from electric shocks. Functional earthing Is designed to ensure optimal performance of the PV array, but it is required only if specified by the manufacturer. Fuse A device that protects conductors from excessive current. The fuse is rated to carry a certain current, and when this current is exceeded the fuse will open the circuit (by melting). Irradiance The total amount of solar radiation available at any point in time per unit area and is measured in W/m2 or kW/m2. It is a measure of power. Irradiation The total amount of solar radiation available per unit area over a specified time period such as one day. It is the sum of irradiance values over a time period and is often measured in kWh/m2/year or MJ/m2/day. It is a measure of energy. Junction box A box containing a junction of electric wires or cables. Kilowatt peak (kWp) Is a non-SI unit used in the solar industry to describe the nominal power of a solar PV system: it refers to the peak output under standard test conditions. Low Voltage (LV) Electrical systems that operate over 120 V DC (ripple free) or 50 V AC. Systems of these voltages require an electrical license to operate or install. Maximum Power Is the point on the I–V curve that gives the maximum power. It occurs when the load Point (MPP) resistance is equal to the internal resistance of the PV cell. Maximum Power An electronic device included within the inverter that alters the PV array’s electrical Point Tracker (MPPT) output so that it is performing at the maximum power possible at any given time. Module inverter An inverter that is designed to be mounted on the back of a module. Module efficiency The amount of electrical power produced by the module per amount of light energy hitting the module. This is typically lower than cell efficiency due to losses from reflection from glass etc. Monocrystalline The most efficient and most expensive solar cells. They have a smooth solar cells monochromatic appearance. Multicrystalline solar Less efficient solar cells, but cheaper to make and buy. They have a ‘glittering’ effect cells (polycrystalline) when in sunlight. Multi-string inverter An inverter with multiple MPPT’s (e.g. one MPPT per string). Nominal Operating The photovoltaic cell junction temperature corresponding to nominal operating Cell Temperature conditions in a standard reference environment of 800 W/m2 irradiance, 20° C (NOCT) ambient air temperature, 1 m/s wind speed and electrically open circuit.

5 Open Circuit An open circuit is where the current path is broken so that the current is equal to 0. Peak Sun Hours Is a unit of energy used in the solar industry when measuring irradiation. 1 PSH = 1 (PSH) kW of solar energy falling on a surface of 1 m2 for 1 hour. Photovoltaic (PV) A device which creates electricity when sunlight hits its surface. Power Power is the rate at which electric energy is transferred. Power is measured in watts (W), and is calculated using P = V x I PV array Strings of PV modules are electrically connected in parallel to form an array. Also called a solar array. PV cell A single PV device. Also called a solar cell. PV module PV cells are physically and electrically connected to form a PV module. These cells are held together by a frame and covered by a protective substance such as glass. Also called a solar module. PV string When PV modules are connected in series they form a string. PV sub-array Very large PV arrays are often made up of many smaller PV arrays known as sub- arrays. PV system The PV array and all associated equipment required to make it work. Also called a solar electric system. Regulators Devices used to control the charging current to a battery to make sure that the battery is not overcharged. Resistance Is the opposition to current and is measured in ohms (Ω). Root mean square Is how AC power is usually quoted; for example: VRMS = 0.707 × VP ... IRMS = (RMS) 0.707 × IP Self Discharge Rate The rate at which a battery discharges when it is idle. As a battery ages its self discharge rate generally increases. Short Circuit A short circuit is where the current is flowing in a closed path across the source terminals. Solar Altitude The angle between the sun and the horizon. This angle is always between 0° and (Elevation) Angle 90°. Solar Azimuth Angle The angle between north and the point on the compass where the sun is positioned on a horizontal plane. The azimuth angle varies as the sun moves from east to west across the sky throughout the day. In general, the azimuth is measured clockwise going from 0° (true north) to 359°. Solar Cell Is a small photovoltaic unit that generates an electrical current when hit by sunlight. Solar Modules See PV module. Solar radiation Energy coming from the Sun. Specific Gravity This is the ratio of the density of the battery electrolyte with respect to water. Hydrometers, which sample the electrolyte, are used to monitor the state of charge of the batteries. Stand Alone Power Systems designed to provide electrical power at a site which is remote from the Systems (SAPS) electricity grid. The system generally gets its’ energy from renewable energy sources (solar, wind, water) and is often backed up by a mechanical generator (petrol, diesel, LP gas, steam). Standard Test Standardised test conditions which make it possible to conduct uniform comparisons Conditions (STC) of PV modules by different manufacturers. State of Charge The available capacity in a cell or battery expressed as a percentage of rated capacity. String inverter An inverter with only one MPPT. Surge capacity The capacity of an inverter to deliver power at a higher rate than its rated power output for given short periods( measured in seconds) of time. Switch-disconnector Is a mechanical switching device capable of making, carrying and breaking currents

6 in normal circuit conditions and, when specified in given operating overload conditions. In addition, it is able to carry, for a specified time, currents under specified abnormal circuit conditions, such as short-circuit conditions. Moreover, it complies with the requirements for a disconnector. Temperature The amount by which the capacity of a battery should be adjusted to account for Correction Factor changes in temperature. Battery capacities are usually given at a reference (Batteries) temperature of 25° C. Temperature The amount by which the voltage, current or power from a solar cell will change Correction Factor with changes in the temperature of the cell. (Solar Cells) Thin film solar cells Made from materials that are suitable for deposition on large surfaces such as glass. Very thin in comparison to monocrystalline and multicrystalline solar cells. Least efficient technology, but the cheapest to manufacture. Voltage Is the potential difference between two points. Voltage is measured in volts (V).

7 Introduction This set of guidelines sets out best practice for the design, installation and maintenance of Standalone Photovoltaic Systems (PV). These systems are typically installed in areas where the grid is not available. However these guidelines could also be applied to systems that are used as a back-up system for a building, which is connected to the grid. The output of such a system either feeds directly to circuits, which are not connected to the grid or connects to circuits during a grid failure via a changeover switch.

The guidelines cover following system types:

• Stand alone PV systems (d.c. bus) with d.c. only output • Stand alone PV systems (d.c. bus) with a.c. only output

The guidelines are divided into three parts as follows:

Part 1: System Design

Part 2: System Installation

Part 3: System Maintenance

System Design outlines the best practice processes undertaken when determining why a system is required and then selecting and matching the equipment that will be part of a standalone PV system to meet specified design requirements. It provides the formulas used to match components and for determining the energy output of the system. Once the modules, controllers, batteries and inverter(if included) have been determined, international and local standards are applied to determine and select the balance of system requirements such as cables, isolation and protection devices.

System Installation outlines the best practices when following international and local standards to install the standalone PV system.

System Maintenance outlines the best practices that should be employed on maintaining the standalone PV system to prolong the lifetime of the system as well as to reduce system losses.

8 Part 1: System Design This set of guidelines provides the minimum knowledge required to design a Standalone PV system using the d.c. bus configuration. A stand-alone system will be designed:  Either as a full system kit that provides the end user with the system equipment including lights and small DC appliances that will operate for a specified number of hours per day, a small Solar Home System);  Or according to the actual energy needs of the end-user as determined by having completed a load assessment for the site.

The design of any Standalone PV system should consider the electrical load as well as a number of criteria, including:  Budget  Power quality  Environmental impact  Aesthetics  Site accessibility

Having applied the comprehensive design criteria, a designer shall use this information to:  Determine the size of the PV array (kWp) to meet the daily energy requirements;  Select the appropriate system d.c. voltage;  Determine the controller size and type based on the size of the array;  Determine the battery capacity;  Determine all the balance of equipment.

1.1 ENERGY SOURCE MATCHING Heating and lighting should be supplied from the most appropriate source. For example o Cooking - gas or wood burning stove or wood burning/charcoal etc o Water heating - solar water heating with gas or wood backup o Lighting - electrical lighting most often used but natural light (daylighting) should be considered.

1.2 ENERGY EFFICIENCY All appliances should be chosen for the lowest possible energy consumption for each desired outcome, such as o High efficiency lighting o Energy efficient refrigeration

1.3 STANDARDS FOR DESIGN System designs should follow any standards that are typically applied in the country or region where the solar installation will occur. The following are the relevant standards

9 for Kenya. These standards are often updated and amended so the latest version should always be applied.

The following Kenyan standards are applicable:

KS 1672 Glossary of terms and symbols for solar photovoltaic power generation KS 1673-1 Solar photovoltaic power systems – design, installation, operation, monitoring and maintenance KS 1673-2 Generic specification for solar photovoltaic systems – system design, installation, operations, monitoring and maintenance KS 1674 Crystalline silicon terrestrial photovoltaic (PV) modules – design qualification and type approval KS 1675 Thin-film terrestrial photovoltaic (PV) modules – design qualification and type approval KS 1676 Terrestrial photovoltaic (:PV) power generating systems – general and guide KS 1677 Procedures for temperature and irradiance corrections to measured I-V characteristics of crystalline silicon photovoltaic devices KS 1678 Photovoltaic devices KS 1679 UV test for photovoltaic (PV) modules KS 1680 Overvoltage protection for photovoltaic (PV) power generating systems – guide KS 1681 Characteristic parameters of standalone photovoltaic systems KS 1682 Salt mist corrosion testing of photovoltaic (PV) modules KS 1683 Rating of direct coupled photovoltaic (PV) pumping systems KS 1684 Susceptibility of a photovoltaic (PV) module to accidental impact damage (resistance to impact test) KS 1685 Photovoltaic system performance monitoring – guidelines for measurement, data exchange and analysis KS 1686 Analytical expression for daily solar profiles KS 1709-1 Batteries for use in photovoltaic power systems – General requirements KS 1709-2 Batteries for use in photovoltaic power systems – Modified lead acid batteries KS 1709-4 Batteries for use in photovoltaic power systems – Recommended practice for sizing lead acid batteries for photovoltaic (PV) systems

The following international standards (IEC) are applicable:

IEC 60364-5 Wiring rules IEC 62548 Photovoltaic (PV) arrays – design requirements IEC 62109 Safety of power converters for use in photovoltaic power systems IEC 62446 Grid connected photovoltaic systems – Minimum requirements for system documentation, commissioning tests and inspection IEC 62305 Protection against lightning IEC 61730.1 Photovoltaic module safety qualification : requirements for construction

10 IEC 61730.2 Photovoltaic module safety qualification : requirements for testing IEC 61683 Photovoltaic systems ; procedure for measuring efficiency IEC 61215 Crystalline silicon terrestrial photovoltaic (PV) modules – Design qualification and type approval IEC 61646 Thin-film terrestrial photovoltaic (PV) modules - Design qualification and type approval IEC 61427 Secondary cells and batteries for renewable energy storage - General requirements and methods of test - Part 1: Photovoltaic off- grid application

1.4 OVERVIEW OF SYSTEM DESIGN

Four major issues arise when designing a system:

1. The load (power) required to be supplied by the system is not constant over The period of one day; 2. The daily energy usage varies over the year; 3. The energy available from the PV array may vary from time to time during the day; 4. The energy available from the PV array will vary from day to day during the year.

Since the system is based on photovoltaic modules, then a comparison should be undertaken between the available energy from the sun and the actual energy demands. The worst month is when the ratio between solar energy available and energy demand is smallest.

The design of a standalone PV system requires a number of steps. A basic design method follows:

1. Determination of the energy usage that the system must supply. 2. Determination of the battery storage required. 3. Determination of the energy input required from the PV array. 4. Selection of the remainder of system components.

1.5 LOAD (ENERGY) ASSESSMENT

Electrical power is supplied from the batteries (d.c.) or via an inverter to produce 240 volts a.c.. Electrical energy usage is normally expressed in watt hours (Wh) or kilowatt hours (kWh).

To determine the daily energy usage for an appliance, multiply the power (in Watts) of the appliance by the number of hours per day it will operate. The result is the energy (Wh) consumed by that appliance per day.

Appliances can either be d.c. or a.c.. An energy assessment should be undertaken for each type, examples of these are shown in annexures 5 and 6

11 You need to calculate the electrical energy usage with the customer. Many systems have failed over the years not because the equipment has failed or the system was installed incorrectly, BUT BECAUSE THE CUSTOMER BELIEVED THEY COULD GET MORE ENERGY FROM THEIR SYSTEM THAN THE SYSTEM COULD DELIVER. It failed because the customer was unaware of the power/energy limitations of the system.

The problem is that the customer may not want to spend the time determining their realistic power and energy needs which are required to successfully complete a load assessment form. They just want to know: How much for a system to power my lights and TV?

A system designer can only design a system to meet the power and energy needs of the customer. The system designer must therefore use this process to understand the needs of the customer and at the same time educate the customer. Completing a load assessment form correctly (Refer to annexures 5 and 6 to complete the load assessment forms and learn about the design process) does take time; you may need to spend 1 to 2 hours or more with the potential customer completing the tables. It is during this process that you will discuss all the potential sources of energy that can meet their energy needs and you can educate the customer on energy efficiency.

1.6 SOLAR HOME SYSTEM DESIGN METHODOLOGY (d.c) Though the intention of this section is to describe sizing your small d.c. solar home system that is generally provided as a kit complete with module, battery, controller and lights (possibly radio and TV) the same methodology can be applied to large systems with d.c. only loads.

1. Load Assessment

The small solar home system (less than 100Wp) is generally designed to meet the lighting load with the possibility of small radio or small d.c. powered TV.

The steps involved in estimating the load are:

1. List all the items in the household, which would draw electricity from the batteries.

2. Determine the rated power of each of these items.

3. Estimate the number of hours per day that the item would be used.

4. Multiply the power (in watts) by the number of hours to get the energy used by each in a unit called watt-hours (watt-hrs or Wh) item.

5. Add up the answers.

A sample load assessment table has been shown in annex 5 of this guideline.

2. Battery Selection

12 Once the total energy required per day has been determined it can be converted into Ampere-Hours, commonly called amp-hrs or abbreviated to Ah, which is the usual unit used to measure battery capacity. The system voltage of a solar home system is typically 12 Volts (V).

To convert watt-hrs (Wh) to Amp-hrs (Ah) divide by the battery system voltage (V). This is obtained from the formula:

Power (watts) = voltage (V) x current (A).

A sample battery selection table has been shown in annex 5 of this guideline.

3. Days of Autonomy

The battery bank capacity is determined by: 1. Multiplying the daily Ah demand by the number of days of autonomy required 2. Then dividing this number by the maximum depth of discharge allowed.

Autonomy is the maximum number of days that the batteries can supply the daily demand, when there has been no input from the energy source that is PV module.

In Solar home systems the recommended number of days of autonomy is between 3 and 5 days. This term must be explained to the customer.

4. Maximum Depth of Discharge

The battery manufacturers will specify the maximum allowable depth of discharge. That is the depth that the battery can be taken before the battery will be damaged. This therefore provides the capacity that can be taken out of the battery to supply the loads before this point is reached. The manufacturer’s depth of discharge can vary in the range 0.5 - 0.8 (50 to 80%).

Typically for 12V batteries used in solar home systems it will be 70% or 0.7. (note: If car batteries are used than the maximum depth of discharge should be 50%)

For Kenya it is recommended that for solar batteries in solar home systems under 100Wp: • The days of autonomy is 3 days • The maximum depath of discharge is 70%

A sample comprising of days autonomy and Maximum Depth of Discharge has been shown in annex 5 of this guideline.

5. Temperature Correction

The next step is to divide by a temperature correction factor. The storage capacity depends on the temperature of the batteries and even though care is taken to keep the batteries at the optimum temperature it is safer to make allowance for variations.

13 The capacity of the batteries is rated at 25°C, figure 1 shows the effect on temperature on the capacity. In Kenya the night time temperature varies greatly between the regions at altitude to those regions located in the hotter lower altitudes So it is important that the lowest temperature for the actual location of the selected site is used. If the system is being installed into regions where the temperature can get very cold then the temperature derating must be determined from figure 1.

A sample showing temperature correction has been shown in annex 5 of this guideline.

6. Discharge Rates

Battery suppliers specify the Ah capacities of batteries, at particular discharge rates. In

SHS with 5 days autonomy and small loads, we generally recommend the use of the C 20 discharge rate.

One of the batteries that is typical in SHS in Kenya is 100Ah at C 20. This battery has an effective capacity of 100 Ah at a discharge current of 5 amps (100Ah/20h).

7. Required Battery capacity and number of batteries

Please remember that it is important to minimise the number of parallel strings in the battery bank for the following reasons:

 With parallel strings there may be the chance of uneven charging of the batteries in the bank. The batteries closest to the charging source may charge at a marginally higher rate than those further from the source.

 When checking the state of charge of the batteries with a hydrometer, each cell of the battery must be inspected. If, for example, the battery bank consisted of three, 12 volt batteries in three parallel strings there would be 18 cells to check.

The number of batteries in parallel is determined by the overall capacity divided by the capacity of the selected battery.

Note: Battery manufacturers typically only allow a maximum of 3 or 4 batteries in parallel

A sample for getting the required battery capacity through paralleling has been shown in annex 5 of this guideline.

8. Average daily depth of discharge

To obtain long life from the battery then the daily depth of discharge should be less than 20 to 25%.

14 To meet a design criteria of 5 days autonomy down to 70% depth of discharge then the daily depth of discharge should be 14%.

To meet a design criteria of 3 days autonomy down to 70% depth of discharge then the daily depth of discharge should be 23.3%.

A sample for average daily depth of discharge below 14% has been shown in annex 5 of this guideline.

9. Daily Energy requirement for the battery from the PV array

The photovoltaic modules must provide an amount of energy at least equal to that consumed by the load so that the batteries do not run out of charge.

Also, because the batteries are not 100% efficient, the array must produce more than the daily requirement. To determine the actual required array output we divide the daily energy requirement by the battery round trip efficiency. This is usually a figure between 0.80 and 0.90 and depends on the efficiency of the batteries in both charging and discharging. Use 0.90 for very efficient batteries installed in good conditions and 0.8 for the least efficient batteries.

It is recommended that efficiency between 0.8 and 0.9 be used for SHS.

A sample for daily energy requirement for the battery from the PV array has been shown in annex 5 of this guideline.

10. Derating the module performance for current

The PV module will be de-rated due to: a. Manufacturers Output Tolerance

The output of a PV module is specified in watts and with a manufacturing tolerance based on a cell temperature of 25 degrees Celsius. Historically this has been 5%, but in recent years typical figures have been -0% and +5%. When designing a system it is important to incorporate the actual figure for the selected module. b. Derating Due to Dirt

The output of a PV module can be reduced as a result of a build-up of dirt on the surface of the module. The actual value of this derating will be dependent on the actual location; some city locations might have a high dirt derating due to car pollution, some coastal locations might have a high dirt derating due to salt build up and some locations might have long periods with no rain to naturally wash the modules.

If in doubt, an acceptable dirt derating value would be 5%.

c. Derating Due to Temperature

15 A solar modules output power decreases with temperature above 25°C and increases with temperatures below 25°C. The average cell temperature will be higher than the ambient temperature because of the glass on the front of the module and the fact that the module absorbs some heat from the sun. The output power and/or current of the module must be based on the effective temperature of the cell. This is determined by the following formula:

Tcell-eff = Ta.day + 25°C

Where:

 Tcell-eff = the average daily effective cell temperature in degrees Celsius (°C)

 Ta.day = the daytime average ambient temperature for the month that the sizing is being undertaken.

Since the modules are used for battery charging, the current at the charging voltage at the effective cell temperature should be used in calculations. For a 12V battery, a charging voltage of 14V is appropriate. If curves are unavailable to determine the current at effective cell temperature then use the Normal Operating Cell temperature (NOCT) provided by the manufacturers.

Therefore the derated module output current is calculated as follows:

The Current of the module at 14V and effective cell temperature (or NOCT current) multiplied by derating due to manufacturers tolerance multiplied by derating due to dirt

I (NOCT) x fman x fdirt If a module has a 3% (0.03) manufacturer’s tolerance, then the module current is derated by multiplying by 0.97 (1-0.03).

If a module has a 5% (0.05) loss due to dirt then the module current is derated by multiplying by 0.95 (1-0.05).

A sample for derating the module performance for current has been shown in annex 5 of this guideline.

11. Oversize factor Since the small solar home system does not include a fuel generator which can provide extra charging to the battery bank then the solar array should be oversized to provide the equalisation charging of the battery bank. It is recommended in Kenya that this is 10%.

A sample calculation with oversize factor has been shown in annex 5 of this guideline.

12. Number of Modules required for the system

The typical small solar home system uses a 12V battery and hence will be connected to solar modules that are nominally 12V (36 cells)

16 To determine the number of modules in parallel, the PV array output current required (in A) is divided by the output of each module (in A). Then round up to the next whole number.

A sample for finding the number of modules required in a small solar home system has been shown in annex 5 of this guideline.

Note: Many modules on the market today are designed for the grid connected (grid tie market) and designed to be interconnected with grid tie inverters. These modules are typically not 36 cell modules. Their number of cells can vary from 48 cells to 96 cells however most are 60 cell modules. These modules are not designed specifically for charging batteries but can be used for charging batteries if a maximum power point charge controller is used (refer section 1.7.3)

1.7 DESIGN METHODOLOGY (a.c. loads) This section is for stand alone PV systems that provide power to a.c. loads via an inverter. In reality the system might also supply power to d.c. loads and a.c loads. If the system includes d.c. and a.c. loads than then the two load assessment forms provided in annex 5 and 6 must be completed and when determining the energy that is supplied by the battery the energy required for the d.c. loads is added to the energy calculated required to meet the a.c. loads through the inverter. It is this combined total that is then used to calculate the size of the array.

1.7.1 Load Assessment and Battery determination 1. Load Assessment

Load assessment is done in a similar way as that for a d.c system. But for an a.c. system we also calculate the continuous VA demand and the surge demand to make sure that the inverter can handle the a.c. loads safely.

A sample load assessment table has been shown in annex 6 of this guideline. 2. Inverter Efficiency

For a.c. systems, we have to take into account the efficiency of the inverter. Typically the peak efficiency of the inverter may be over 90% but in many systems the inverter will sometimes be running when there is very little load on the inverter, so the average efficiency is about 85% to 90%. Then we must divide the total a.c. energy used by this figure to obtain the energy required to be supplied to the inverter from the battery bank.

A sample calculation with inverter efficiency has been shown in annex 6 of this guideline.

System voltages are generally 12, 24 or 48 Volts. The actual voltage is determined by the requirements of the system. For example, if the batteries and the inverter are a long way from the energy source then a higher voltage may be required to minimise power loss in the cables. In larger systems 120V ,240V d.c. or higher could be used, but these are not typical residential or small commercial systems.

17 As a general rule, the recommended system voltage increases as the total load increases. For small daily loads, a 12V system voltage can be used. For intermediate daily loads, 24V is used and for larger loads 48V is used.

The changes over points are roughly at daily loads of 1 kWh and 3-4 kWh but this will also be dependent on the actual power profile.

One of the general limitations is that maximum continuous current being drawn from the battery should not be greater than 150A.

To convert Watt-hours (Wh) to Amp-hours (Ah) you need to divide by the battery system voltage.

Battery capacity is determined by whichever is the greater of the following two requirements:

1. The ability of the battery to meet the energy demand of the system, often for a few days, sometimes specified as ‘days of autonomy’ of the system; OR 2. The ability of the battery to supply peak power demand.

The critical design parameters include:

Parameters relating to the energy requirements of the battery: a) Daily energy demand b) Daily and maximum depth of discharge c) Number of days of autonomy

Parameters relating to the discharge power (current) of the battery: a) Maximum power demand b) Surge demand

Parameters relating to the charging of the battery: a) Maximum Charging Current

Based on these parameters there are a number of factors that will increase the battery capacity in order to provide satisfactory performance. These correction factors must be considered.

A sample calculation with battery selection has been shown in annex 6 of this guideline.

4. Days of Autonomy

18 Extra capacity is necessary where the loads require power during periods of reduced input. The battery bank is often sized to provide for a number of days autonomy. A common period selected is 5 days.

Often where there is no auxiliary charging source (e.g fuel generator with a battery charger), the period of autonomy is often increased to 7 days or more.

For Kenya it is recommended that 5 days is used for system above XX Wp

A sample calculation with days of autonomy has been shown in annex 6 of this guideline.

5. Maximum Depth of Discharge

The battery manufacturers will specify the maximum allowable depth of discharge. That is the depth that the battery can be taken before the battery will be damaged. This therefore provides the capacity that can be taken out of the battery to supply the loads before this point is reached. The manufacturer’s depth of discharge can vary in the range 0.5 - 0.8 (50 to 80%).

A sample calculation with maximum depth of discharge has been shown in annex 6 of this guideline.

6. Battery Discharge Rate

The actual discharge rate selected is highly dependent on the power usage rates of connected loads. Many appliances operate for short periods only, drawing power for minutes rather than hours. This affects the battery selected, as battery capacity varies with discharge rate. Information such as a power usage profile over the course of an average day is required for an estimate of the appropriate discharge rate.

For small systems this is often impractical.

The C100 (100hr discharge rate) capacity rating of the battery could be used for systems where 5 days autonomy has been used in determining the battery capacity however where average power usage rates are high, it may be necessary to select the battery capacity at a higher discharge rate. eg. the 10hr (C10) or 20hr (C20) rate.

A sample calculation with battery discharge rate has been shown in annex 6 of this guideline.

7. Battery Temperature derating

Battery capacity is affected by temperature. As the temperature goes down, the battery capacity reduces. Figure 1 (in the d.c system) shows a graph which gives a battery correction factor for low temperature operation. Note that the temperature correction factor is 1 at 25°C as this is the temperature at which battery capacity is specified.

In Kenya the night time temperature varies greatly between the regions at altitude to those regions located in the hotter lower altitudes So it is important that the lowest

19 temperature for the actual location of the selected site is used. If the system is being installed into regions where the temperature can get very cold then the temperature derating must be determined from figure 1. . A sample calculation with battery temperature rating has been shown in annex 6 of this guideline.

Deep discharge type batteries / cells should be selected for the required system voltage and capacity in a single series string of battery cells.

Parallel strings of batteries are not recommended but if it is unavoidable then they should be kept to a minimum. (Note: Battery manufacturers typically only allow a maximum of 3 or 4 batteries in parallel)

Where this is necessary each string must be separately fused.

A sample calculation with battery selection has been shown in annex 6 of this guideline.

1.7.2 PV ARRAY SIZING- Standard Switched Controllers

The calculation for determining the size of the PV array is dependent on the type of controller used. Historically standard switched controllers were the most common controllers used. In recent years a number of maximum power point trackers (MPPT) have become available. This section determines how to size the PV array based on switched controllers based on the PV array meeting the daily load requirements all year. Section 1.7.3 details how to size a PV array using a MPPT.

The size of the solar array should be selected to take account of:  Seasonal solar radiation data for selected tilt angle and orientation, taking shading into account  Variation of daily/seasonal energy usage  Battery efficiency  Manufacturing tolerance of modules  Temperature effects on the modules  Effects of dirt on the modules  System losses (eg power loss in cables)  Inverter efficiency

Solar irradiation data is available from various sources; some countries have data available from their respective meteorological department. One source for solar irradiation data is the NASA website: http:/eosweb.larc.nasa.gov/sse/. RETSCREEN, a program available from Canada that incorporates the NASA data, is easier to use. Please note that the NASA data has, in some instances, had higher irradiation figures than that recorded by ground collection data in some countries. But if there is no other data available it is data that can be used.

Solar irradiation is typically provided as kWh/m2 . However it can be stated as daily peak Sunhrs (PSH). This is the equivalent number of hours of solar irradiance of 1kW/m2.

20 Annexure 1 provides PSH data on the following sites:

 Mombasa (Latitude 04°03′S Longitude 39°40′E)  Nairobi (Latitude 01°17' S' Longitude 36°49' E)  Wajir (Latitude 01° 45' N Longitude 40°03 E)  Lodwar (Latitude 03°07'N, Longitude 35°36'E)

The variation of both the solar irradiation and the load energy requirement should be considered. If there is no variation in daily load between the various times of the year then the system should be designed on the month with the lowest irradiation that is peak sun hours (PSH).

Note: PV standalone systems can be mounted on the roof of a building. The roof might not be facing true north (Southern hemisphere) or south (northern hemisphere) or at the optimum tilt angle. The irradiation data for the roof orientation (azimuth) and pitch (tilt angle) shall be used when undertaking the design. Please see the following discussion on tilt and orientation for determining peak sun hours for sites not facing the ideal direction.

EFFECT OF ORIENTATION AND TILT When the roof is not orientated true north (southern hemisphere) or south (northern hemisphere) and/or not at the optimum inclination the output from the array will be less than the maximum possible.

Annex 4 provides a diagram showing estimated tilt and orientation losses for a location with a latitude of 1°N.

The table provides values for orientations at 5° intervals (azimuths) and inclination angles at 10° intervals.

Using this figure will provide the system designer/installer with information on the expected output of a system (with respect to the maximum possible output) when it is located on a roof that is not facing true north (for the southern hemisphere) or south (for the northern hemisphere), or at an inclination equal to the latitude angle. The designer can then use the peak sun hour data for their particular location to determine the expected peak sun hours at the orientation and tilt angles for the system to be installed.

1. Daily Energy Requirement from the PV Array

In order to determine the energy required from the PV array, it is necessary to increase the energy from the battery bank to account for battery efficiency.

The average columbic efficiency (in terms of Ah) of a new battery is 90% (variations in battery voltage are not considered).

A sample calculation with daily energy requirement from the PV array has been shown in annex 6 of this guideline.

21 2. Oversize Factor

If the system does not include a fuel generator which can provide extra charging to the battery bank then the solar array should be oversized to provide the equalisation charging of the battery bank. It is recommended in Kenya that this is 10%.

3. Derating Module Performance

The PV array will be de-rated due to: a. Manufacturers Output Tolerance

The output of a PV module is specified in watts and with a manufacturing tolerance based on a cell temperature of 25 degrees Celsius. Historically this has been 5%, but in recent years typical figures have been -0% and +5%. When designing a system it is important to incorporate the actual figure for the selected module. b. Derating Due to Dirt

A solar modules output power decreases with temperature above 25°C and increases with temperatures below 25°C. The average cell temperature will be higher than the ambient temperature because of the glass on the front of the module and the fact that the module absorbs some heat from the sun. The output power and/or current of the module must be based on the effective temperature of the cell. This is determined by the following formula:

Where:

Since the modules are used for battery charging, the current at the charging voltage at the effective cell temperature should be used in calculations. For a 12V battery, a charging voltage of 14V is appropriate. If curves are unavailable to determine the current at effective cell temperature then use the Normal Operating Cell temperature (NOCT) provided by the manufacturers.

22 Therefore the derated module output current is calculated as follows:

The Current of the module at 14V and effective cell temperature (or NOCT current) multiplied by derating due to manufacturers tolerance multiplied by derating due to dirt

I (NOCT) x fman x fdirt

If a module has a 3% (0.03) manufacturers tolerance, then the module current is derated by multiplying by 0.97 (1-0.03).

A sample calculation with derating module performance has been shown in annex 6 of this guideline.

4. Number of Modules required in the Array

i. First determine number of modules in series, to do this divide the system voltage by the nominal operating voltage of each module. In our example:

ii. To determine the number of strings in parallel, the PV array output current required (in A) is divided by the output of each module (in A). Then round up to the next whole number.

A sample calculation with number of modules required in the array with a standard series controller has been shown in annex 6 of this guideline.

5 CONTROLLERS- Standard Switched Controller

PV controllers on the market range from simple switched units that only prevent the overcharge (and discharge) of connected batteries to microprocessor based units that incorporate many additional features such as … o PWM and equalisation charge modes o DC Load control o Voltage and current metering o Amp-hour logging o Generator start/stop control

Unless the controller is a model that is current limited these should be sized so that they are capable of carrying 125% of the array short circuit current and withstanding the open circuit voltage of the array. If there is a possibility that the array could be increased in the future then the controller should be oversized to cater for the future growth.

(Note: sometimes the controller is called a regulator)

A sample calculation with switch controller in the array has been shown in annex 6 of this guideline.

23 1.7.3 PV ARRAY SIZING- MPPT

Please refer to start of section 1.7.2 for information on solar irradiation for Kenya.

1. Daily Energy Requirement from the PV Array

The size of the PV array should be selected to take account of: . seasonal variation of solar irradiation . seasonal variation of the daily energy usage . battery efficiency (wh) . Cable losses . MPPT efficiency . manufacturing tolerance of modules . dirt . temperature of array (the effective cell temperature)

With the standard controller the only sub-system losses was the battery efficiency and the calculations are undertaken using Ah. When using a MPPT the calculations are in Wh and the sub-system losses in the system include:

 Battery efficiency (watt-hr)  Cable losses  MPPT efficiency

In order to determine the energy required from the PV array, it is necessary to account for all the sub-system losses. The energy required at the battery in Wh is then divided by these three losses to determine the required energy to be provided by the array.

A sample calculation with daily energy requirement from the PV array has been shown in annex 6 of this guideline.

2. Oversize Factor

If the system does not include a fuel generator which can provide extra charging to the battery bank then the solar array should be oversized to provide the equalisation charging of the battery bank. It is recommended in Kenya that this is 10%.

3. Derating Module Performance

The PV array will be de-rated due to: a. Manufacturers Output Tolerance

The output of a PV module is specified in watts and with a manufacturing tolerance based on a cell temperature of 25 degrees Celsius. Historically this has been 5%, but in

24 recent years typical figures have been -0% and +5%. When designing a system it is important to incorporate the actual figure for the selected module.

b. Derating Due to Dirt

A solar modules output power decreases with temperature above 25°C and increases with temperatures below 25°C. The average cell temperature will be higher than the ambient temperature because of the glass on the front of the module and the fact that the module absorbs some heat from the sun. The output power and/or current of the module must be based on the effective temperature of the cell. This is determined by the following formula:

Where:

The three different solar modules available on the market each have different temperature coefficients. These are:

A) Monocrystalline Modules Monocrystalline Modules typically have a temperature coefficient of –0.45%/oC. That is for every degree above 25oC the output power is derated by 0.45%.

B) Polycrystalline Modules Polycrystalline Modules typically have a temperature coefficient of –0.5%/oC.

C) Thin Film Modules Thin film Modules have a different temperature characteristic resulting in a lower co-efficient typically around 0%/°C to -0.25%/°C, but remember to check with the manufacturer

The derating of the array due to temperature will be dependent on the type of module installed and the average ambient maximum temperature for the location.

The typical ambient daytime temperature in many parts of Kenya is between 25 and 40oC during some times of the year. So it would not be uncommon to have module cell temperatures of 65oC or higher.

25 With switched controllers the temperature effect was used to determine the operating current of the module/array. With MPPT’s the temperature derating power factor must be calculated.

Therefore the derated module output power (Pmod) is calculated as follows:

The Power of the module at STC multiplied by derating due to manufacturers tolerance multiplied by derating due to dirt multipled by derating due to temperature

Pstc x fman x fdirt x ftemp

If a module has a 3% (0.03) manufacturers tolerance, then the module current is derated by multiplying by 0.97 (1-0.03).

If a module has an ambient temperature of 30 degrees and a temperature coefficient of -0.5% than it has a 15% (0.05) loss due to temperature (0.5%x(30+25-25)) then the module current is derated by multiplying by 0.85 (1-0.15).

4. Number Of Modules Required In Array

To calculate the required number of modules in the array, divide the required array power by the adjusted (derated) module power.

The exact final number required will depend on the MPPT selected and then matching the array to the MPPT voltage operating windows

A sample calculation with number of modules required in the array with an MPPT controller has been shown in annex 6 of this guideline

5. Selecting MPPT

The output voltage of the MPPT shall match the battery voltage selected.

The maximum input power rating of the MPPT shall be equal to or greater than the rated power of the array.

The maximum input current rating of the MPPT must be equal to or greater than the rated current of the array. Note that the current of the array will be dependent on the number of parallel strings while the number of parallel strings will be dependent on the number of modules in series in each string. The number of modules in series must match the operating voltage window of the MPPT as detailed below.

Matching The PV Array To The Voltage Specifications Of The MPPT

The MPPT typically have a recommended minimum nominal array voltage and a maximum voltage. In the case where a maximum input voltage is specified and the array voltage is above the maximum specified, the MPPT could be damaged.

26 Some MPPT controllers might allow that the minimum array nominal voltage is that of the battery bank. However the MPPT will work better when the minimum nominal array voltage is higher than the nominal voltage of the battery. Please check with the MPPT manufacturer because these could vary.

It is important that the output voltage of the string is matched to the operating voltages of the MPPT and that the maximum voltage of the MPPT is never reached.

The output voltage of a module is affected by cell temperature changes in a similar way to the output power. The manufacturers will provide a voltage temperature coefficient. It is generally specified in V/°C (or mV/°C) but it can also be expressed as a %.

To ensure that the Voc of the array does not reach the maximum allowable voltage of the MPPT the minimum day time temperatures for that specific site are required.

In early morning at first light the cell temperature will be very similar to the ambient temperature because the sun has not had time to heat up the module. In Kenya Islands the average minimum temperature is 200C (this could be lower in some mountain areas) and it is recommended that this temperature is used to determine the maximum Voc. (Note: If installing in the mountains then use the appropriate minimum temperature. Many people also use 0°C, if appropriate for the area). The maximum open circuit voltage is determined similar to the temperature derating factor for the power.

A sample calculation with MPPT controller sizing has been shown in annex 6 of this guideline.

1.7.4 Inverter Selection The type of inverter selected for the installation depends on factors such as cost, surge requirements, power quality and for inverter/chargers, a reduction of the number of system components necessary.

Inverters are available in 2 basic output types - modified square (or sine) wave and sine wave.

Modified square (sine) wave inverters generally have good surge and continuous capability and are usually cheaper than sine wave types. However, some appliances, such as audio equipment, television and fans can suffer because of the output wave shape.

Sine wave inverters often provide a better quality power than the 240V grid supply.

If affordable to the end-user yt is recommended that sine waves inverters are used.

The selected inverter should be capable of supplying continuous power to all AC loads AND providing sufficient surge capability to start any loads that may surge when turned on and particularly if they turn on at the same time.

Where an inverter cannot meet the above requirements attention needs to be given to load control and prioritisation strategies.

A sample calculation with Inverter selection has been shown in annex 6 of this guideline.

27 1.8 CABLE SELECTION-ARRAY

Figure 1: Double insulated solar d.c. cable

Cables used within the PV array shall:  Be suitable for d.c. application,  Have a voltage rating equal to or greater than the PV array maximum voltage determined in table 1.  Have a temperature rating according to the application.  If exposed to the environment, cables should be UV-resistant , or be protected from UV light by appropriate protection, or installed in UV-insulated conduit,  Be water resistant.  if exposed to salt environments be tinned copper, multi stranded conductors to reduce degradation of the cable over time,  In all systems operating at voltages above DVC-A, cables shall be selected so as to minimise the risk of earth faults and short-circuits. This is commonly achieved using reinforced or double-insulated cables, particularly for cables that are exposed or laid in metallic tray or conduit. This can also be achieved by reinforcing the wiring.  Cables shall be flame retardant as defined in IEC60332-1-2.  It is recommended that string cables be flexible (class 5 of IEC60228) to allow for thermal/wind movement of arrays/module.  Cables should comply with PV1-F requirements or UL 4703 or VDE-AR-E-2283-4.

Note: PV1-F cable requirements may be found in the document TUV 2 PfG 1169/08.2007.

Correctly sized cables in an installation will produce the following outcomes:- 1. There is no excessive voltage drop (which equates to an equivalent power loss) in the cables. 2. The current in the cables will not exceed the safe current handling capability of the selected cables known as current carrying capacity (CCC)

Selection PV String Cables  If a fault current protection device is located in the string, then the string must be rated to carry at least that current. For example, if the fault current protection device is rated at 8A, then the string will need to be rated at a minimum of 8A.  If no fault current protection is provided, then the string cable will be rated as:

CCC ≥ 1.25 × ISC MOD × (Number of Strings - 1)

Selection of PV Array Cables  The PV array cable should be rated according to:

CCC ≥ 1.25 × ISC ARRAY and  have a CCC equal to or greater than the rating of the array protection device.

28 There are a large number of other system configurations possible for a standalone PV system which have their own specific requirements. The following explains the requirements of some of the more complicated systems.

1.8.1 Sub-array PV Systems A sub-array comprises a number of parallel strings of PV modules. The sub-array is installed in parallel with other sub-arrays to form the full array. The effect of this is to decrease the potential fault current through different parts of the system. In this case the following modifications will be necessary.

PV Array Cables  The PV array cable should be rated according to:

CCC ≥ 1.25 × ISC ARRAY and  have a CCC equal to or greater than the rating of the array protection device.

PV Sub-array Cables  If a fault current protection device is located in the sub-array cable, the sub- array cable must have a rating equal to or greater than that of the fault current protection device.  If no fault current protection device has been included then the current carrying capacity of the cable must be the greater of:

1.25 × (sum of short circuit currents of all other sub‐arrays)

1.25 ×ISC Sub-Array PV String Cables  If sub-array fault current protection is used, the string cable rating will be the rated trip current of the sub-array fault current device plus the fault current of the other stings in the sub-array:

Itrip-subarray + 1.25 × ISC MOD × (Number of Strings - 1)

 If no sub-array fault current protection device is used, the string cable rating will be: 1.25 × (sum of short circuit currents of all other strings in the array):

1.9 CABLE PROTECTION-Array

All cables shall be electrically protected from fault currents that could occur. In stand alone systems these fault currents will be provided by the battery bank so that all array cables shall require protection. The array cable will therefore require protection at the controller end of the array cable and this protection will typically be provided as a non- polarised circuit breaker (d.c. rated) that also acts as the array disconnector (refer section 1.11).

29 Sub-array and string cable protection will be required if the current carrying capacity of these cables is less than the current rating of the array cable protection device. However if sub-array and string cable protection is not required due to fault currents from the battery they might still be required because of the currents from the solar modules as detailed below.

Each solar module has a maximum reverse current rating provided by the manufacturer. If the arrays consists of parallel strings such that the reverse current flow into a string with a fault is greater than the maximum reverse current for the modules in that string then protection shall be provided in each string. The protection are to be d.c. rated fuses.

Example: The reverse current rating for a module is 15A while the short circuit current is 5.4A. If the array consists of 3 parallel strings and a fault occurs in one string then the potential fault current will come from the other 2 strings which is only 10.8A (2 x 5.4) and is less than the reverse current rating so no protection is required. However if the array consists of 4 parallel strings then the fault current could come from the other 3 strings. This current is 16.2 A (3 x 5.4) and is now greater than the reverse current rating of the module. Protection is now required.

A formula for determining the maximum number of strings allowed before fuses is required is:

Maximum Number of Strings without string protection

= reverse current rating of module/Isc of module

So in the above example; Max Number of strings = 15/5.4= 2.77 rounded off to 3.

Fuses Fuses used in PV arrays shall— (a) be rated for d.c. use; (b) have a voltage rating equal to or greater than the PV array maximum voltage determined via table 1; (c) be rated to interrupt fault currents from the PV array,; and (d) be of an overcurrent and short circuit current protective type suitable for PV complying with IEC 60269-6 (i.e. Type gPV).

String Protection

The fuses shall have the following current rating:

1.5 x Isc of module < Fuse Rating < 2.4 x Isc of module

Sub-Array Protection

An array may be broken up into sub-arrays for different reasons; for example, if two sections of the array are installed in separate areas. The need for sub-array overcurrent protection is similar in logic to that for string overcurrent protection – one sub-array could be operating differently from the other sub-arrays owing to shading or earth faults. The use of sub-array protection is to stop excessive currents from flowing into a sub-array.

30 Requirements of Sub-array Overcurrent Protection Sub-array overcurrent protection protects a sub-array made up of a group of strings. It is required if one of the following conditions is met:

 1.25 × ISC_ARRAY > Current carrying capacity (CCC) of any sub-array cable, switching and connection device.  More than two sub-arrays are present within the array.

Sizing the Sub-array Overcurrent Protection If sub-array overcurrent protection is required for a system, the nominal rated current for the overcurrent protection device will be as follows:

Where:

ISC_SUB-ARRAY = short-circuit current of the sub-array.

ITRIP = rated trip current of the fault current protection device.

Array Cable Protection

Array overcurrent protection is designed to protect the entire PV array cabling from external fault currents in the presence of batteries as we have in the Standalone PV systems.

Sizing the Array Overcurrent Protection

If array protection is required, the rated current of the overcurrent protection device will be the following:

Where:

ISC_ARRAY = the short-circuit current of the array.

ITRIP = rated trip current of the overcurrent protection device. 1.10 CABLE SELECTION-Voltage Drop Cable losses between the PV array and the controller (standard or MPPT) and battery bank should be as low as practical, consistent with cable size and cost decisions. Cable losses between the inverter and battery should ensure that the inverter does not trip due to low battery voltage when inverter is operating at continuous rating. The following should be applied:

 Cable losses between the PV array and the battery bank should never exceed 5%  Cable losses between the PV array and the controller should never exceed 3%  Cable losses between the battery bank and any DC load should never exceed 5%  Cable losses between the battery bank and inverter should never exceed 5%.

The following sizing methods (based on voltage drops) can be used for all types of currently available copper cable.

1. Calculating Voltage Drop 31 Voltage drop is calculated using Ohm’s law:

Combining this with the formula for calculating resistance, the voltage drop along a cable is given by:

Voltage drop (in percentage) = × 100 Where:

 LCABLE = route length of cable in metres (multiplying by two adjusts for total circuit wire length).  I = current in amperes *†.  ρ = resistivity of the wire in /m/mm2 2  ACABLE = CSA of cable in mm . ‡  VMAX = maximum line voltage in volts . Note: For systems using standard series controller ,

• the ISC current (at STC) should be used and

• VMAX is equal to the battery nominal voltage. For systems using MPPTs as the controller ,

• the IMP current (at STC) should be used and not the ISC current and

• VMAX is equal to the MPP voltage of the string or array at STC (VMP_STRING or VMP_ARRAY).

For voltage drop between the inverter and battery bank , • the continuous current (d.c.) of the inverter should be used

• VMAX is equal to the battery nominal voltage_).

2. Using Tables - 1 Voltage drop in volts per 10 metres of route length of twin cable ( using the above formula )

Wire size mm² 2 3.2 5 7.5 15 Amps CCC 15 20 25 45 70

0.5 0.09 0.06 0.04 0.02 0.01 1.0 0.18 0.11 0.07 0.05 0.02 1.5 0.27 0.17 0.11 0.07 0.04 2.0 0.37 0.23 0.15 0.10 0.05 2.5 0.46 0.29 0.18 0.12 0.06 3.0 0.55 0.34 0.22 0.15 0.07 4.0 0.73 0.46 0.29 0.20 0.10 5.0 0.92 0.57 0.37 0.24 0.12 7.5 1.37 0.86 0.55 0.37 0.18 10 1.83 1.14 0.73 0.49 0.24 15 2.75 1.72 1.10 0.73 0.37 20 2.29 1.46 0.98 0.49 25 1.83 1.22 0.61 30 1.46 0.73

32 40 1.95 0.98 50 1.22

Notes : Cable size and CCC from Pirelli automotive data Shaded areas indicate that the CCC is exceeded Refer also, to PV module and Inverter manufacturers' recommendations.

3. Using Tables - 2

Route lengths to produce 5% voltage drop (12V systems) for twin cable ( using the above formula )

Maximum Distance in metres to produce 5% voltage drop (12V system) Current (A) 1mm2 1.5mm2 2.5mm2 4mm2 6mm2 10mm2 16mm2 1 16.4 24.6 41 65.6 98.4 163.9 262.3 2 8.2 12.3 20.5 32.8 49.2 82 131.1 3 5.5 8.2 13.7 21.9 32.8 54.6 87.4 4 4.1 6.1 10.2 16.4 24.6 41.0 65.6 5 3.3 4.9 8.2 13.1 19.7 32.8 52.5 6 2.7 4.1 6.8 10.9 16.4 27.3 43.7 7 2.3 3.5 5.9 9.4 14.1 23.4 37.5 8 2.0 3.1 5.1 8.2 12.3 20.5 32.8 9 1.8 2.7 4.6 7.3 10.9 18.2 29.1 10 1.6 2.5 4.1 6.6 9.8 16.4 26.2 11 1.5 2.2 3.7 6.0 8.9 14.9 23.8 12 1.4 2.0 3.4 5.5 8.2 13.7 21.9 13 1.9 3.2 5.0 7.6 12.6 20.2 14 1.8 2.9 4.7 7.0 11.7 18.7 15 1.6 2.7 4.4 6.6 10.9 17.5 16 1.5 2.6 4.1 6.1 10.2 16.4 17 2.4 3.9 5.8 9.6 15.4 18 2.3 3.6 5.5 9.1 14.6 19 2.2 3.5 5.2 8.6 13.8 20 2.0 3.3 4.9 8.2 13.1

1.11 Main Battery Cable Selection and Protection

Overcurrent protection and the ability to readily isolate a battery bank must be provided.

33 1.11.1 d.c. Only systems Determine the maximum discharge current that would be required to provide power to all the lights and d.c. appliances that will be on at any one time. For small systems this will include all lights and appliances, however for some systems with many loads it will be those determined as being the maximum demand.

The protection device (typically a fuse) should be selected to provide as a minimum this current while the cable size should be capable of carrying that current and as a minimum the current rating of the protection device and have a voltage drop less than the maximum allowable voltage drop specified in section 1.10.

1.11.2 a.c. systems

Some inverters are supplied with battery cables already connected while for those not supplied with cable the size should either be determined by the designer or the manufacturers recommendations should be followed.. As a minimum the cable shall be capable of carrying the d.c. current required to provide the continuous power rating of the inverter. Note if the inverter does have a 1/2 hour power than this should rating be used for determining the minimum cable size.

The cable should have a voltage drop less than the maximum allowable voltage drop specified in section 1.10. However inverters have a surge rating. Since the surge only occurs for less than 3 seconds the selected cable is not required to carry that current on a continuous and should not overheat in that time basis however the required surge current rating should be used to determine that the selected cable will not adversely effect the performance of the inverter when it is providing the surge rating.

To select the appropriate main battery protection …

 Obtain Time-Current characteristics for the over current protection to be used. [All manufacturers publish time-current information for their circuit breaker and HRC fuse ranges]

 Obtain inverter manufacturers data Continuous power rating ( Watts ) 3 to 10 second surge rating ( Watts ) Average inverter efficiency

 For each inverter power rating determine the current drawn from the battery bank using …

I = Inverter Power Rating ( W ) . ( inverter efficiency x nominal battery voltage )

NOTE: Allowance for any significant DC demand must be included when sizing the main protection

 Consult the Time-Current characteristic to determine the appropriate rating.

Since the battery must be capable of being isolated and also cable protection is required a suitable rated d.c switch fuse as shown in figure 4 is often used

34 INSERT PHOTO OF SWITCH FUSE

Figure 2: Battery Swicth Fuse

1.12 PLUGS and SOCKETS

Plugs, sockets and connectors shall— a) comply with EN 50521; b) be protected from contact with live parts in connected and disconnected states(e.g. shrouded); c) have a current rating equal to or greater than the current carrying capacity for the circuit to which they are fitted ; d) be capable of accepting the cable used for the circuit to which they are fitted; e) require a deliberate force to separate; f) have a temperature rating suitable for their installation location; g) if multi-polar, be polarized; h) comply with Class II for systems operating above DVC-C; i) if exposed to the environment, be rated for outdoor use, be of a UV-resistant type and be of an IP rating suitable for the location; j) be installed in such a way as to minimize strain on the connectors (e.g. supporting the cable on either side of the connector); and k) only be mated with those of the same type from the same manufacturer.

1.13 DC Switch disconnector at controller

D.C. switch disconnector at the controller

In accordance with IEC/TS 62548 there must be a method to isolate the power from the array at the controller. This switch disconnector shall be load breaking and break all non-earthed poles. However as specified in section 1.9, protection is also required for protecting the array cable from fault currents from the battery bank.

Where a controller allows more than one input from the array a switch disconnector shall be installed on each input and these should be located physically beside each other near the controller. Signage should indicate that to operate the PV array all switch diconnectors must be operated together.

Note: A switch disconnector not rated for the open circuit d.c. voltage (based on coldest temperature-refer to next section) of the array and 1.25 times the d.c.. short circuit current of the array shall not be used as the PV Array Switch-disconnector.

35 To meet the isolation and protection requirements it is recommended that a non- polarised d.c. rated circuit breaker is installed as close as possible to the controller.

Voltage Limits

System voltage classification has been done as per the DVC as per IEC 62548 standard.

Decisive voltage Limits of working voltage (V) classification (DVC) AC voltage (rms) AC voltage (peak) DC voltage (mean) A V ≤ 25 V ≤ 35.4 V ≤ 60 B 25 ˂V ≤ 50 35.4 ˂V ≤ 50 60 ˂V ≤ 120 C V ˃ 50 V ˃ 71 V ˃ 120

PV Array Maximum Voltage

The PV Array Maximum voltage can be calculated using the minimum expected temperature at a site and the temperature coefficient of a module. This calculation was done in the worked example when calculating the maximum number of modules that can be connected in a string.

If the temperature coefficients are not available, the PV Array Maximum voltage can be determined by using the below table containing the temperature ranges and multiplication factors.

Lowest expected operating Correction factor temperature (degrees Celsius) 24 to 20 1.02 19 to 15 1.04 14 to 10 1.06 9 to 5 1.08 4 to 0 1.10 -1 to -5 1.12 -6 to -10 1.14 -11 to -15 1.16 -16 to -20 1.18 -21 to -25 1.20 -26 to -30 1.21 -31 to -35 1.23 -36 to -40 1.25

Table 1: Voltage correction factors for crystalline and multi-crystalline silicon PV modules

36 Part 2: System Installation

The performance of a reliable installation that fulfils a customer’s expectations requires both careful design and correct installation practice. Compliance with relevant Health and Safety regulations is necessary.

2.1 STANDARDS for INSTALLATION System installs should follow any standards that are typically applied in the country or region where the solar installation will occur. The following are the relevant standards for Kenya. These standards are often updated and amended so the latest version should always be applied.

The following Kenyan standards are applicable:

KS 1672 Glossary of terms and symbols for solar photovoltaic power generation KS 1673-1 Solar photovoltaic power systems – design, installation, operation, monitoring and maintenance KS 1673-2 Generic specification for solar photovoltaic systems – system design, installation, operations, monitoring and maintenance KS 1674 Crystalline silicon terrestrial photovoltaic (PV) modules – design qualification and type approval KS 1675 Thin-film terrestrial photovoltaic (PV) modules – design qualification and type approval KS 1676 Terrestrial photovoltaic (:PV) power generating systems – general and guide KS 1677 Procedures for temperature and irradiance corrections to measured I-V characteristics of crystalline silicon photovoltaic devices KS 1678 Photovoltaic devices KS 1679 UV test for photovoltaic (PV) modules KS 1680 Overvoltage protection for photovoltaic (PV) power generating systems – guide KS 1681 Characteristic parameters of standalone photovoltaic systems KS 1682 Salt mist corrosion testing of photovoltaic (PV) modules KS 1683 Rating of direct coupled photovoltaic (PV) pumping systems KS 1684 Susceptibility of a photovoltaic (PV) module to accidental impact damage (resistance to impact test) KS 1685 Photovoltaic system performance monitoring – guidelines for measurement, data exchange and analysis KS 1686 Analytical expression for daily solar profiles KS 1709-1 Batteries for use in photovoltaic power systems – General requirements KS 1709-2 Batteries for use in photovoltaic power systems – Modified lead acid batteries KS 1709-4 Batteries for use in photovoltaic power systems – Recommended practice for sizing lead acid batteries for photovoltaic (PV) systems

The following international standards (IEC) are applicable:

IEC 60364-5 Wiring rules IEC 62548 Photovoltaic (PV) arrays – design requirements

37 IEC 62109 Safety of power converters for use in photovoltaic power systems IEC 62446 Grid connected photovoltaic systems – Minimum requirements for system documentation, commissioning tests and inspection IEC 62305 Protection against lightning IEC 61730.1 Photovoltaic module safety qualification : requirements for construction IEC 61730.2 Photovoltaic module safety qualification : requirements for testing IEC 61683 Photovoltaic systems ; procedure for measuring efficiency IEC 61215 Crystalline silicon terrestrial photovoltaic (PV) modules – Design qualification and type approval IEC 61646 Thin-film terrestrial photovoltaic (PV) modules - Design qualification and type approval IEC 61427 Secondary cells and batteries for renewable energy storage - General requirements and methods of test - Part 1: Photovoltaic off- grid application

VOLTAGE LIMITS

System voltage classification has been done as per the DVC as per IEC 62548 standard.

Decisive voltage Limits of working voltage (V) classification (DVC) AC voltage (rms) AC voltage (peak) DC voltage (mean) A V ≤ 25 V ≤ 35.4 V ≤ 60 B 25 ˂V ≤ 50 35.4 ˂V ≤ 50 60 ˂V ≤ 120 C V ˃ 50 V ˃ 71 V ˃ 120

2.2 DOCUMENTATION All complex systems require a user manual for the customer. Standalone PV systems are no different. The documentation for system installation that must be provided is …  List of equipment supplied.  Shutdown and isolation procedure for emergency and maintenance.  Engineering certificate for wind and mechanical loading  Installer/designer declaration of compliance with wind and mechanical loading  Operating instructions  Maintenance procedure and timetable.  Commissioning sheet and installation checklist.  Warranty information.  System connection diagram (as installed).  System performance estimate  Equipment manufacturers documentation  Array frame engineering certificate  Array frame installation declaration and  Handbooks for all equipment supplied.

38 2.3 PV MODULES PV modules shall comply with the requirements of IEC 61730-1 and IEC 61730-2, or EN 61730-1 and EN 61730-2, or UL Standard 1703.

2.4 PV ARRAY The installation of the PV Array shall be in accordance with IEC/TS62548.

ORIENTATION AND TILT

In stand alone PV systems the solar array is generally mounted:  “Flat” on the roof (that is parallel to the slope of the roof) OR  On an array frame that is tilted to fix the array at a preferred angle (usually for flat roofs or ground mounted) OR  On a pole.

Modules that are electrically in the same string must be all in the same orientation.

For best year-round performance a fixed PV array should be mounted facing true north ( 10°) in the parts of Kenya that are in the southern hemisphere, and true south ( 10°) in the parts of Kenya that are in the northern hemisphere at an inclination equal to the latitude angle or at an angle that will produce the best annual average performance taking into consideration: seasonal cloud patterns, local shading and environmental factors. In the tropics this could vary due to the sun being both north and south at different times of the year. Note: A minimum tilt of 10° is recommended to take advantage of self-cleaning during rain periods. Horizontally mounted arrays will require additional maintenance [cleaning].

For locations between the latitudes of 10° South and 10°North, the array should be tilted at a minimum of 10 degrees.

39 The optimum tilt angle would be approximately 4° for Mombasa and 3° for Lodwar. However, to account for self-cleaning, the tilt angle of both should be around 10°.

If the array is “flat” on the roof (that is parallel to the slope of the roof) or integrated into the building fabric, the array will often not be at the preferred (optimum) tilt angle and in many situations will not be facing due north or due south.

Annex 3 provides PSH data on the following sites:  Mombasa (Latitude 04°03′S Longitude 39°40′E)  Nairobi (Latitude 01°17' S' Longitude 36°49' E)  Wajir (Latitude 01° 45' N Longitude 40°03 E)  Lodwar (Latitude 03°07'N, Longitude 35°36'E)

Annex 4 provides a diagram showing estimated tilt and orientation losses for a location with a latitude of 1°N.

Using these figures will provide the system designer/installer with information on the expected output of a system (with respect to the maximum possible output) when it is located on a roof that is not facing true north (for the southern hemisphere) or south (for the northern hemisphere), or at an inclination equal to the latitude angle. The designer can then use the peak sun hour data for their particular location to determine the expected peak sun hours at the orientation and tilt angles for the system to be installed.

ROOF MOUNTING  If the modules use crystalline cells then it is preferable to allow sufficient space below the array (> 50mm or 2 inches) for ventilation cooling. This will be subject to the constraints of the customer or architect.  It is important to allow sufficient clearance to facilitate self cleaning of the roof to prevent the build up of leaves and other debris.  If fauna are a problem in the vicinity of the installation then consideration should be given to how to prevent them gaining access under the array.(see cable protection)  All supports, brackets, screws and other metal parts should be of similar material or stainless steel to minimise corrosion. If dissimilar metals (based on their galvanic rating) are used then the two surfaces of the metals should be separated by using rubber washes or similar.  Where timber is used it must be suitable for long-term external use and fixed so that trapped moisture cannot cause corrosion of the roof and/or rotting of the timber. The expected replacement time should be stated in the system documentation.  Any roof penetrations must be suitably sealed and waterproof for the expected life of the system. If this is not possible then this must be detailed in Maintenance Timetable  All fixings must ensure structural security when subject to the highest wind speeds for the region and local terrain - This may require specific tests of the fixing/substrate combination on that roof.  The installer shall ensure that the array frame that they install has applicable engineering certificates verifying that the frame meets wind loadings for that particular location.

40  The installer must follow the array frame suppliers/manufacturers recommendations when mounting the array to the roof support structure to ensure that the array structure still meets wind loading certification.  All external wiring must be protected from UV and mechanical damage in such a manner that it will last the life of the system.(See cable Protection).

FREE STANDING PV ARRAYS These must be wind rated in accordance with relevant wind loading standards

POLE MOUNTED PV ARRAYS For small solar home systems the array could be mounted on a pole. These must be wind rated in accordance with relevant wind loading standards

2.5 OUTDOOR MOUNTED COMBINER BOXES

Arrays that require string, sub-array or array protection should use a string and/or array combiner box to house the system protection and to interconnect the strings and/or sub-arrays. A combiner box can also be used when there is no overcurrent protection required, as it can provide protection for cable interconnections.

The array and/or sub-array combiner box must have an appropriate IP rating for its location: a minimum IP rating of 54 plus UV resistance for outdoor equipment. The manufacturer’s instructions will outline the process for maintaining the IP rating. For example, all cable entries into the combiner box should be through the bottom to prevent water ingress.

Installers may be tempted to drill holes into the box for drainage or ventilation. However, drilling holes into the box will void the IP rating and so should never be done.

Figure 5: Outside combiner box

41 Note: Ensure that the combiner box does not shade the array.

2.6 PV ARRAY SWITCH DISCONNECTOR

The switch-disconnector ( recommended to be a double pole non-polarised d.c. circuit breaker) used should have an appropriate IP rating for its location. However the controller is typically located under cover and hence so will the PV array disconnector.

The switch disconnector should be located beside the controller.

2.7 CONTROLLER INSTALLATION The controller can be located anywhere between the array and the battery bank however it is often located near the battery bank.

The controller should be: Installed in a dust free environment Installed in a location that minimises the controller being in excessive temperatures due to the outside ambient temperature

The controllers' heat sink must be clear of any obstacles to facilitate cooling of the inverter. The manufacturers recommended clearances must be followed.

Since controllers often have screens or Leds providing information to the end user consideration should be given to its location so that it is easiliy accessible by the system owner. 2.8 INVERTER INSTALLATION

The inverter should be: Installed as close as possible to the battery bank to minimise voltage drop. Installed in a dust free environment Installed in a location that minimises the inverter being in excessive temperatures due to the outside ambient temperature Mechanically supported where it is mounted

Since inverters often have screens or Leds providing information to the end user consideration should be given to its location so that it is easily accessible by the system owner.

The inverters' heat sink must be clear of any obstacles to facilitate cooling of the inverter. The manufacturers recommended clearances must be followed.

2.9 BATTERY INSTALLATION

All batteries should be installed in a dedicated enclosure. The enclosure could be a dedicated battery room, a battery box just for the batteries or if installed within a large room or shed than the enclosure could be a fenced off area,

42 Even the single battery small solar home systems should have the battery installed in a dedicated enclosure.

Please refer to figures 5 through to 9 for examples of battery enclosure requirements.

The main considerations for the battery enclosure are … a) A minimum horizontal separation of 500 mm shall be provided between the battery and all other equipment from 100 mm below battery terminals except where there is a solid separation barrier. b) If a battery is separately enclosed in a battery box with no other equipment installed in the box there is no need for 500 mm clearance from the battery to the walls of the battery box. c) Socket-outlets shall not be installed in the battery enclosure. d) Where the battery enclosure is part of a larger room (e.g. the battery enclosure is a fenced off area in a larger room), all socket-outlets shall be located at least 1800 mm from the battery enclosure and a minimum of 100 mm below the top of the battery or any battery vent, if within 5 m of the battery enclosure. e) No equipment shall be placed above the batteries or battery enclosure except for non- metallic battery maintenance equipment. f) A purpose-built equipment enclosure may be installed above a purpose built battery enclosure where all of the following apply: a.i. A sealed (valve regulated) battery is installed in the battery enclosure. a.ii. A gas proof horizontal barrier is in place between the battery enclosure and the equipment enclosure. a.iii. The battery and equipment enclosures are accessed separately (e.g. via separate doors). a.iv. The ventilation paths for the battery enclosure and the equipment enclosure are specifically designed to minimize the possibility of air exhausted from the battery enclosure entering the air inlets on the equipment enclosure g) It should not be in located in direct sunlight. h) Extreme ambient temperatures should be avoided because low temperatures decrease battery capacity and high temperatures shorten battery life i) It must be safe, with restricted access ( ie. prevent children easily accessing the batteries ) j) All equipment must be readily accessible for maintenance k) It must have adequate ventilation l) It should be vermin proof m) Exhaust air shall not pass over other electrical devices. n) Battery enclosures should be designed to prevent formation of gas pockets. o) Ventilation outlets should be at the highest level of battery enclosure. p) The ventilation inlets should be at a low level in the battery enclosure. They should not be higher than the top of the battery cells. q) To ensure airflow does not pass the battery, the ventilation inlets and outlets should be on opposite sides of the enclosure however figure x shows what to do if this is not possible. r) The supporting surface of the enclosure should have adequate structural strength to support the battery bank weight and its support structure. s) The enclosure should be resistant to the effects of electrolyte, either by the selection of materials used or by appropriate coatings. Provision should be made for the containment of any spilled electrolyte. Acid resistant trays in which the

43 batteries sit should be capable of holding electrolyte equal to the capacity of at least one cell of the battery bank. t) Any enclosure doors should allow unobstructed exit

The main safety considerations are …  explosion due to a spark in the presence of hydrogen build up  excessive currents caused by battery shorts  leakage of battery acid from battery cells  personal safety in the presence of acid

To negate the risk of explosion there must be no opportunity for hydrogen to build up. This requires adequate ventilation with no possibility of spark ignition.

Figure 3: Battery installed in a dedicated equipment room showing clearances from equipment

44 Figure 4: Battery enclosure within a room where the battery enclosure is vented outside the building

Figure 5: Battery enclosure with equipment enclosure immediately adjacent

45 Figure 6: Battery enclosure with the intake and outlet vents on the same wall

2.10 VENTILATION OF BATTERIES

Ventilation must be provided. The minimum area required for natural ventilation for both inlet and outlet apertures (for wet lead acid batteries) are given by …

A = 100qv cm²

Where qv is the minimum exhaust ventilation rate in litres per second = 0.006 x n x I and n = the number of battery cells I = the charging rate in amperes

For vented wet lead acid batteries the charging rate in amperes is the maximum output rating of the largest charging source or the rating of its output fuse or circuit breaker. Where two parallel battery banks are used, the charging rate is halved.

For valve regulated (sealed) batteries the charging rate I in the ventilation formula is 0.5A per 100Ah at the 3h rate (C3)of discharge of battery capacity for lead acid batteries.

e.g. battery has C3 rating of 500Ah therefore the charge current used in ventilation formula is : (500Ah/100Ah) x 0.5A = 2.5A

Note; This is based on the charger (either solar controller or separate grid power battery charger) has an automatic overvoltage cut-off. If not maximum change current must be used.

46 Best practice is to provide input ventilation vents below the level of battery and the output vents on the opposite side of the batteries, as high as possible in the enclosure to prevent hydrogen build up.

2.11 PREVENTING SPARK IGNITION SOURCES NEAR BATTERIES a) Electrical equipment or storage for other equipment should not be mounted above the battery bank. b) Connection or disconnection of any equipment at the battery terminals must not occur where there is any possibility of the presence of any hydrogen build up. c) Battery charging equipment should be hard wired, do not use temporary connection. d) Battery terminals should be shrouded to prevent inadvertent short circuits. e) Ensure sufficient clearance between battery terminals and metal walls ( or insulate using non-metallic sheet ) f) maximise separation between battery terminals g) use insulated tools during any battery work

Battery fusing preferably should not be in the same enclosure as the battery bank but if they are then they should be either a minimum of 500mm away from the batteries or 100mm below the top of the batteries.

Another method to keep the fuse separate from the battery bank is to place a vertical partition between the batteries and the fuse, thereby keeping the fuse as close to the batteries as possible but isolated from any hydrogen build up. In any case the main battery fusing should be located below the battery vents. (Normally below the top of the batteries).

2.12 PREVENTING EXCESSIVE CURRENT FROM BATTERIES

Battery shorts are prevented by shrouding terminals and ensuring safe separation between live terminals.

Battery shorts are controlled by using appropriate circuit protection.

Overcurrent protection is to be provided in each battery output conductor except where one side of the battery bank is earthed (ground), in which case only the unearthed (ungrounded) conductor requires overcurrent protection.

Normal practice is to either fuse the positive and earth (ground) the negative or fuse all conductors. 2.13 BATTERY SAFETY AND WARNING SIGNS

A “Battery Explosion Warning" sign must be mounted so that it is clearly visible on approach to the battery bank. An "ELECTROLYTE SAFETY" sign should be mounted adjacent to the battery bank. 2.14 CABLE INSTALLATION All cables shall be installed in a neat and tidy manner and in accordance with IEC 60364-5.

47 All cables used in the installation should be securely fixed in place to minimise any movement of the cable.

Where the cables could be damaged then there should be suitable mechanical protection of the cables.

Where the presence of fauna is expected to constitute a hazard, either the wiring system shall be selected accordingly, or special protective measures shall be adopted.

All conduits exposed to sunlight must be suitably UV rated. Not all corrugated conduits are UV rated so if using corrugated conduit ensure that it is UV rated.

Plastic cable ties are not suitable for cables in exposed situations and should not be used as the primary means of support unless they have a lifetime greater to or equal to the expected life of the system.

Connection of a.c.. and d.c.. components in the same enclosure should be segregated. d.c.. wiring shall not be placed in a.c. switchboards.

To avoid conductive loops the positive and negative cables of an array shall be run in parallel.

Figure 7: Avoiding the conductive loop between the positive and negative cables

When the array mounted on a roof, the solar module interconnect cables must be supported clear of the roof surface to prevent debris build up or damage to insulation.

The installer shall ensure that all module connectors used are waterproof and connected securely to avoid the possibility of a loose connection. Only connectors of the same type from the same manufacturer are allowed to be mated at a connection point.

All unprotected and unearthed battery cables— (1.a) not exceeding 2 m in length; (1.b) used for the connection of a battery terminal and the battery overcurrent

48 device; and (1.c) contained in a battery box, battery room or in a fenced off area specifically allocated for batteries do not require additional mechanical protection.

For battery cables exceeding 2 m, the cables shall be protected by PVC conduit or equivalent protection.

If the unprotected cable between the battery terminals and battery overcurrent device leave the battery box, battery room or in a fenced off area they shall be mechanically protected by PVC conduit or equivalent.

2.15 WIRING OF ARRAYS WITH VOLTAGE ABOVE DVC-A

A dangerous situation occurs when the person installing the system is able to come in contact with the positive and negative outputs of the solar array or sub-array when the output voltage is above DVC-A Most standalone systems use approved solar modules which are connected using double insulated leads with polarised shrouded plug and socket connections. Therefore the dangerous situation is only likely to occur for systems using MPPT's at :  the PV Array switch-disconnector before the controller for systems; AND  the sub-array and array junction boxes (if used). To prevent the possibility of an installer coming in contact with live wires on systems using MPPT's and voltage is above DCV-A it is recommended practice that one of the interconnect cables of each string (as shown in Figure 11) is left disconnected until all the wiring is complete between the array and the inverter. Only after all switch- disconnectors and other hard wired connections are completed should the interconnect of the array be connected.

Figure 8: Disconnected interconnect cable

2.16 WIRING FROM ARRAYS WITH VOLTAGES ABOVE DCV-A TO PV ARRAY SWITCH-DISCONNECTOR NEAR CONTROLLER

The PV array cable shall be clearly identified as d.c. solar cable to ensure that it cannot be mistaken for a.c. cable.

49 Between the array and the controller single core double insulated solar cable is used. This cable is similar to that used for interconnecting the solar modules in the array.

PV d.c. cables between the array and the inverter shall be installed in conduit to reduce the risk of short circuit.

2.12 EARTHING OF ARRAY FRAMES (PROTECTIVE EARTH/GROUND)

For systems with a voltage above DVC-A all non-electrical conductive parts of the system, such as the module frames and the mounting system, shall be equipotentially bonded. It is important to use appropriate methods for equipotential bonding, with consideration given to the potential of galvanic corrosion between galvanically dissimilar metals.

Earth conductors should be in close proximity to the main PV array positive and negative conductors. The earth conductor shall pass in close proximity to the Inverter and then follow the output conductors of the Inverter, where possible, to avoid electrical interference generated by the inverter propagating to other parts of the system.

The bonding earth should be connected to the main earth conductor without interrupting the conductor. That is one module can be removed without affecting the protective bonding of other modules in the array.

There are two ways of doing this with:  Specifically designed stainless steel washers that penetrate the non-conductive coating of aluminum frames and bond solar PV modules to the mounting structure and thereby create an electrical path to earth.(See figure 12 and 13).  By using a lug on each module and then use earth wire that is continuous (no daisy chaining) between the modules.(Refer figure 14).

All earth connections to the mounting rail should be sprayed with corrosion-resistant paint to protect the connection from corrosion from the weather.

Figure 9: Stainless steel earthing washers

50 Figure 10: Earthing washer (WEEB) under the mid-clamp

Figure 11: Earth lugs used for earthing the modules.

51 The earth bonding cable shall be a minimum of 4 mm2 unless it is also required for lightning protection and then it shall be a minimum of 16 mm2 (Refer Figure 9 of IEC TS 62548 for further information)

2.15 SIGNAGE All circuits, protective devices, switches and terminals shall be suitably labelled and all signs and labels shall be suitably affixed and durable

 There should be a sign on the switchboard stating what is the maximum d.c.. array short circuit current and array open circuit voltage from the system.  Any junctions boxes used between the array and the inverter should have a sign “Solar d.c..” on the cover. All d.c junction boxes shall carry a warning label indicating that active parts inside the boxes are fed from a PV array and may still be live after isolation from the PV inverter and public supply  A single line wiring diagram shall be displayed on site  Emergency shutdown procedures shall be displayed on site  A “Battery Explosion Warning" sign must be mounted so that it is clearly visible on approach to the battery bank. An "ELECTROLYTE SAFETY" sign should be mounted adjacent to the battery bank.

2.16 COMMISSIONING

Included with this guideline is an installation checklist (Annex 1) which can be used by the installer when they have completed the installation to ensure they have met these guidelines.

The commissioning checklist provided (Annex 2) with these guidelines shall be completed by the installer. A copy shall be provided to the customer in the system documentation and a copy retained by the installer.

52 Part 3: System Maintenance

A standalone PV system that has been correctly installed and commissioned should operate with minimal intervention, however a PV system and batteries does need regular inspection and maintenance to ensure that it is operating efficiently, find any problems and to maximise its lifetime. The suggested system maintenance can be broken down into three components: PV array maintenance, inverter maintenance and balance of system maintenance. 3.1 PV Array Maintenance The PV modules and the mounting system should be inspected every 12 months. The modules should be cleaned of any dirt or debris and any vegetation shading the array should be trimmed.

Visual Inspection The modules should be checked annually for any visible defects, such as module yellowing, micro-fractures and hot spots (Figure 19).

Figure 12 - : a) A yellowed module, b) micro-fractures in a module and c) a module hotspot.

Mounting System Inspection The mounting system should be checked annually to make sure that the modules are securely mounted and the mounting system is suitably attached to the roof. The mounting system should also be checked for any cracks, corrosion or signs of weakening.

Module Cleaning The modules may need to be cleaned manually if they are installed in a dusty area, if there has been a prolonged period without rain or if they were installed at a small tilt angle (less than 10°). This Some principles for cleaning PV modules are as follows: PV modules should be cleaned using only water: no detergents, solvents, caustic solutions or acid wash should be used. Abrasive materials should not be used to clean the modules; a soft broom can be used to remove loose debris and gently scrub away harder soiling, such as bird droppings. Leaf matter or animal nests should be gently cleared from beneath the array. Avoid hosing or brushing any electrical cable junctions, combiner boxes, disconnectors, etc. The module manufacturer may provide instructions on recommended cleaning practices for their modules. Vegetation Management

53 Any vegetation that is shading the modules should be trimmed back. This is especially relevant for ground-mounted arrays, as they tend to be closer to the ground and surrounded by vegetation. Grazing animals could be kept around a ground-mounted array to keep vegetation levels down. 3.2 Controller Maintenance Inverters require very little ongoing maintenance: Keep the unit clean to minimise the possibility of dust ingress. Clean the controller housing when required. Ensure the unit is free of insects and spiders. Ensure all electrical connections and cabling are kept clean and tight.

It is recommended that the system owner checks the controller' display every couple of days to check that the PV system is working correctly. 3.3 Battery Bank

The batteries are the most maintenance intensive component in a standalone power system. Please remember that batteries are dangerous so ensure that all tools are suitable for undertaking maintenance and that the room is well ventilated and that there is no build-up of hydrogen before entering the room.

The following maintenance should be undertaken at reasonable intervals.

Battery maintenance should be undertaken at least every 6 months with the following exceptions:

1. After the initial installation, it is recommended that the Specific Gravity readings (for wet lead acid battery installations) are taken monthly to ensure that the system is charging the batteries adequately. Once the customer is satisfied then this could be undertaken along with all the other maintenance.

2. Some wet lead acid batteries might require that the electrolyte level is checked monthly or quarterly.

Battery Bank maintenance should include:

Read and record electrolyte density - specific gravity (flooded batteries) Check and record cell voltage Check electrolyte level, top up where necessary - record water usage Check all battery connections and cable terminations for security and corrosion Check for mechanical damage to battery cells or cases Clean batteries and battery area

A sample of a maintenance log sheet is shown in Table 2.

Date Date Date Date

Battery Voltage

54 Ambient Temperature Cell 1 S G

Electrolyte Temperature

Corrected SG

Cell Volts

…… Cell x S G

Electrolyte Temperature

Corrected SG

Cell Volts

Interconnections OK?

Battery Cases OK?

Comments

Table 2: Example Battery Log Sheet 3.4 Inverter Maintenance Inverters require very little ongoing maintenance: Keep the unit clean to minimise the possibility of dust ingress. Clean the inverter housing and air filters when required. Ensure the unit is free of insects and spiders. Ensure all electrical connections and cabling are kept clean and tight.

3.5 Balance of System Maintenance The balance of system equipment should be inspected annually to ensure that everything is mechanically secure and there are no signs of damage. The inspection should cover: Cables and cable clips Conduit Earth connections Disconnectors Circuit breakers and fuses Signage.

In addition, disconnection devices and circuit breakers should be checked for correct operation and system signage should be checked for visibility.

55 During the inspection, the system’s battery voltage and array current should be measured and recorded.

It is recommended that all module string connections and all other connections on the DC cables between the array , the controller and the batteries and the batteries to the inverter are visually inspected for damage and loose connections.

If possible, it is advisable to use a thermal imaging (i.e. infrared) camera for undertaking part of this inspection. In large systems where there are multiple combiner boxes, taking a thermal image of each junction box could help find loose connections before they become a safety issue.

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56 Annex 1: System Installation Checklist

Item No Type of Item PV module 1 PV Module - Model XYZ-90 2 Series combination of modules 3 Parallel combination of modules 4 Solar Mounting structure 5 Hardware for connecting module to frame Hardware for connecting frame to roof (if 6 required) Solar Controller 7 Cable between module & solar regulator 8 Conduit 9 Fastening hardware for cable/conduit 10 Solar Regulator - Model ABC-40 11 Hardware for fastening controller to wall Fuse/Circuit breaker between solar 12 module/controller Batteries 13 Batteries - Model BBB 400 14 Timber (if battery floor mounted) 15 Battery racks/stands (if required) 16 Battery Box (if required) 17 Coverings for terminals (if required) 18 Cable between controller and battery 19 Lugs or fasteners for cable connection to battery 20 Battery fusing 21 Adequate Battery enclosure is provided Inverter 22 Inverter - Model INV-5000P Inverter Power rating (Continuous , ½ minute 23 and Surge) 24 Transformer / Transformerless 25 Cable between batteries and inverter 26 Cable between charger and batteries 27 Lights for shed/battery room 28 Light Switches 29 Cable between controllers/batteries and lights 30 Fastening hardware for lights/switches 31 Fastening hardware for lighting cable Installation Tools 32 (recommend technician prepares a list) Compulsory Safety Equipment: 33 Safety Goggles 34 Leather Gloves 35 Water Washing bottle 36 Bicarbonate Soda 37 Water Bucket

57 ______Authorisation

I, …………………………………… acknowledge that the following system has been installed to the standards indicated by these guidelines Name of the person for whom the system was installed: …………………………………………………………………………… Location of system: …………………………………………………………… ………………

Signed: ………………………………………………………………… Date: …../…../….. Licence/Qualification number: …………………………………………

Attach a separate sheet detailing any departures

58 Annex 2: Testing and Commissioning Checklist

TO BE DEVELOPED

59 Annex 3: Table showing Peak Sun hours for various sites and tilt angles Location A u Annual Mombasa Feb Mar Apr May Jun g Sep Oct Nov Dec Average 4. 5.98 5.62 5.09 4.53 4.44 8 5.39 5.48 5.47 5.54 5.21 Latitude: 4°03′ South 0 4. 6.02 5.62 5.14 4.62 4.56 8 5.41 5.50 5.53 5.63 5.27 Longitude: 39°40′ East 7 4. 5.99 5.42 5.19 4.82 4.85 9 5.31 5.41 5.58 5.76 5.34 9

A u Annual Nairobi Feb Mar Apr May Jun g Sep Oct Nov Dec Average 5. 6.77 6.57 5.76 5.29 5.05 4 6.29 6.05 5.51 5.98 5.85 Latitude: 1°17′ South 8 5. 6.79 6.58 5.77 5.32 5.08 5 6.29 6.06 5.53 6.01 5.87 Longitude: 36°49′ East 0 5. 6.88 6.42 5.85 5.61 5.46 6 6.20 6.06 5.69 6.30 6.02 7 A u Annual Wajir Feb Mar Apr May Jun g Sep Oct Nov Dec Average 5. 6.43 6.09 5.54 5.38 4.94 3 5.79 5.41 5.02 5.36 5.52 Latitude: 1°45′ North 1 5. 6.46 6.10 5.55 5.40 4.96 3 5.79 5.43 5.04 5.40 5.54 Longitude: 40°03′ East 2 5. 6.68 6.02 5.54 5.55 5.16 3 5.65 5.50 5.29 5.81 5.68 7

A u Annual Lodwar Feb Mar Apr May Jun g Sep Oct Nov Dec Average 6. 6.50 6.31 5.72 5.71 5.58 0 6.53 5.99 5.61 5.80 5.96 Latitude: 03°07′ North 8 6. 6.59 6.33 5.74 5.76 5.65 1 6.53 6.04 5.71 5.94 6.03 Longitude: 35°36′ East 2 6. 6.81 6.23 5.67 5.87 5.84 1 6.29 6.13 6.03 6.43 6.16 4 Annex 4: Tilt and Orientation diagram for 1°N. NW NE 320 325 330 335 340 345 350 355 N (0) 5 10 15 20 25 30 35 40 (315) (45) %LOSS 90 5% 310 80 50 10% 305 70 55 15% 300 60 60 20% 295 50 65 25% 290 40 70 30% 285 30 75 35% 280 20 80 40% 275 10 85 45% W E (90) (270) 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 50% 265 10 95 55% 260 20 100 60% 255 30 105 250 40 110 245 50 115 240 60 120 235 70 125 230 80 130 90 S SE SW 220 215 210 205 200 195 190 185 (180 175 170 165 160 155 150 145 140 (135) (225) )

Annex 5: Solar Home System Example (d.c.)

Table 3: DC Load (energy) Assessment

(1) (2) (3) (4a) (5a) (4b) (5b) (6) Comments dry season wet season Contribution Power to maximum Appliance Numbe Usage Energy Usage Energy demand r Time Time W h Wh h Wh W LED Light 4 7 4 112 4 112 28 TFT-LCD 1 15 2 30 2 30 15 TV Mobile 1 5 3 15 3 15 5 Charger Daily Load energy-d.c loads (Wh) 157 (DC 157 ( DC 7a) 7b) Maximum d.c. demand (W) 48 (DC 8) Battery Selection What is the required Ah demand on the batteries?

Using figure 2, we select a 12V system.

This means that the daily Ah demand on the batteries will be: Wh  system voltage 157Wh  12 = 13.083 Ah Ah =

Days of Autonomy What is the adjusted battery capacity taking inverter efficiency into account? Assume that 5 days autonomy is required.

Adjusted battery capacity = 13.083 Ah x 5 = 65.415 Ah.

Maximum DoD What is the adjusted battery capacity taking DoD into account? Assume that the maximum DoD is 70%.

Adjusted battery capacity = 65.415 Ah / 0.7 = 93.45 Ah.

Battery Discharge Rate What is the required battery discharge rate?

Required battery discharge rate = 93.45 Ah @ C20

Battery Temperature Derating What is the adjusted battery capacity taking temperature into account? Assume the cell temperature is mostly above 20°C. This temperature would not affect the battery capacity.

Battery selection What is the size of the battery that should be selected?

Assume battery capacity is 100Ah at C20 rating.

Number of batteries required to meet the design specification = 93.45 / 100 =0.9345 (round up to the nearest whole number, in this case 1).

Note: Always round the number of parallel stings up, not down.

Therefore the effective battery bank is:

Effective capacity of battery x Number of batteries in parallel = 100 Ah x 1 = 100 Ah

Average daily depth of discharge

What is the daily depth of discharge in relation to the battery capacity? To obtain long life from the battery then the daily depth of discharge should be less than 20%.

To meet the design criteria of 5 days autonomy down to 70% depth of discharge then the daily depth of discharge should be 14%.

Average daily depth of discharge = Daily load = 13.083 = 13.1 % Battery capacity 100

Daily Energy Requirement for the Battery from the PV Array What is the required current from the PV array? Assume the average coulombic efficiency of the battery is 90%. Assume the lowest PSH over the year is 5.

13.083 Ah / 0.9 = 14.54 Ah

Required PV array output current = 14.54 / 5 (Lowest PSH)= 2.91 A

Oversize Factor Assume an oversize factor of 10%

Required PV array output current = 2.91 A x 1.1 = 3.2 A

Derating Module Performance for Current What is the derated current output of the PV module with the specifications provided? Assume a dirt derating of 5%. 60 W Polycrystalline module data Rated Power 60W Power Tolerance ± 3% Nominal Voltage 12V

Maximum Power Voltage, Vmp 17.3V

Maximum Power Current, Imp 3.47A

Open Circuit Voltage, Voc 21.8V

Short Circuit Current, Isc 3.8A NOCT 47±2°C Current at 14V and NOCT 3.54

Module current = 3.54 x 0.97 x 0.95 = 3.26A

Number of Modules Required in the Array What is the required number of modules to charge the battery? What is the peak rating of the array?

Number of modules in series = 12V (battery voltage)/12V (module voltage) = 1

Number of modules in parallel = 3.2 / 3.26A = 0.98 rounded up to 1

Number of modules in array = 1 x 1 = 1.

So 1 module of 60 W will be more than sufficient as per the design specification. Annex 6: Large system example (a.c.)

Table 4: AC Load (energy) Assessment

(1) (2) (3) (4a) (5a) (4b) (5b) (6) (7) (8) (9a) (9b) dry season wet season Contribution Powe to max Appliance No. r Power demand Surge Factor Factor Potential Design Comments Usage Energy Usage Energy Time Time

W h Wh h Wh VA VA VA TV 100 3 300 3 300 0.8 125 4 500 125

Refrigerator 100 12 1200 12 1200 0.8 125 4 500 500 Duty cycle of 0.5 included

Daily Load Energy A.C 1500 1500 Loads (Wh) (AC10b)

maximum demand (VA) (AC11) 1000

Surge demand (VA) (AC12) Inverter Efficiency What is the total load as seen by the battery? Assume the efficiency of the chosen inverter is 90%. 1500Wh  0.9 = According to table 4, the daily battery load (energy) from AC loads = 1667 Wh

Therefore the total load (energy) as seen by the battery = 1667 Wh.

Battery Selection What is the required Ah demand on the batteries?

Using figure 2, we select a 24V system.

This means that the daily Ah demand on the batteries will be: Wh  system voltage 1667Wh  24 = 69.46 Ah Ah =

We can round this up to 70 Ah

Days of Autonomy What is the adjusted battery capacity taking inverter efficiency into account? Assume that 5 days autonomy is required.

Adjusted battery capacity = 70Ah x 5 = 350Ah.

Maximum DoD What is the adjusted battery capacity taking DoD into account? Assume that the maximum DoD is 70%.

Adjusted battery capacity = 350Ah / 0.7 = 500Ah.

Battery Discharge Rate What is the required battery discharge rate?

Required battery discharge rate = 500Ah @ C100

Battery Temperature Derating What is the adjusted battery capacity taking temperature into account? Assume the cell temperature is mostly above 20°C.

This temperature would not affect the battery capacity.

Battery selection What is the size of the battery that should be selected? 500Ah @ C100

STANDARD CONTROLLER Daily Energy Requirement for the Battery from the PV Array What is the required current from the PV array? Assume the average coulombic efficiency of the battery is 90%. Assume the lowest PSH over the year is 5.

70Ah / 0.9 = 77.78Ah

Required PV array output current = 77.78 / 5 = 15.56A

Oversize Factor for Current What is the oversized required PV array output current? Assume an oversizing factor is 10%.

Adjusted Required PV array output current = 15.56A x 1.1 = 17.11A

Derating Module Performance for Current What is the derated current output of the PV module with the specifications provided? Assume a dirt derating of 5%. 80 W Polycrystalline module data Rated Power 80W Power Tolerance ± 3% Nominal Voltage 12V

Maximum Power Voltage, Vmp 17.6V

Maximum Power Current, Imp 4.55A

Open Circuit Voltage, Voc 22.1V

Short Circuit Current, Isc 4.8A NOCT 47±2°C Current at 14V and NOCT 4.75A

Module current = 4.75 x 0.97 x 0.95 = 4.38A

Number of Modules Required in the Array What is the required number of modules to charge the battery? What is the peak rating of the array?

Number of modules in series = 24V (battery voltage)/12V (module voltage) = 2

Number of modules in parallel = 17.11A / 4.38A = 3.91 (round up to 4).

Number of modules in array = 2 x 4 = 8.

Peak rating = 8 modules x 80Wp = 640Wp

Charge Controller What is the required current rating of the controller?

Current rating > 1.25 x 4 strings x 4.8A (Isc) Current rating > 24A. MPT Controller Daily Energy Requirement from the Array What is the daily energy requirement from the array? Assume cable losses are 3%, the efficiency of the MPPT is 95% and the battery efficiency is 80%. Assume the lowest PSH over the year is 5.

Subsystem efficiency = 0.97 x 0.95 x 0.8 = 0.74

Load energy from battery bank= 1667Wh

Energy required from the PV array = 1667Wh / 0.74 = 2253Wh

Required output power = 2253Wh / 5PSH = 451Wp

Oversize Factor for Power What is the oversized required PV array output power? Assume an oversizing factor is 10%.

Adjusted Required PV array output power= 451Wp x 1.1 = 496Wp

Derating Module Performance for Power What is the derated power output of the PV module? Assume a dirt derating of 5%, a manufacturer’s tolerance of ±3% and an ambient temperature of 35°C. Remember, the module is polycrystalline, so the temperature derating factor is approximately 0.5%/°C.

Effective cell temperature = 35°C + 25°C = 55°C Therefore this is 35°C above the STC temperature.

Losses due to temperature = 35°C x 0.5%/°C = 0.175. This is a temperature derating factor of 0.825.

Adjusted module power = 80 x 0.825 x 0.97 x 0.95 = 60.8W

Number of Modules Required in the array What is the required number of modules? What is the peak rating of the array?

Required number of modules = 496 / 60.8 = 8.2. (Round up to 9)

Peak rating = 9 x 80Wp W=720Wp.

Selecting the Charge Controller Which charge controller should be selected? Use table below to make the selection. Allow for a 125% oversizing.

Adjusted array rating = 1.25 x 720 = 900Wp

From the table the most appropriate charge controller is the Outback Flexmax60,

Model d.c. battery Input Max d.c. Max(W) Max Load Voltage (V) voltage Battery Solar Array Current range (V) Current (A) (A) STECA 12/24 17 to 100 20 250/500 10 Solarix MMP2010 Phocos 12/24 Max 95 20 300/600 10 MMPT 100/20-1 Morningstar 12/24 Max 75 15 200/400 15 SS-MPPT- 15L Outback Flex 12/24/36/48/6 Max 150 80 1250(12) Max 80 0 2550(24) 5000(48) 7500 (60) Outback Flex 12/24/36/48/6 Max 150 60 900(12) Max60 0 1800(24) 3600(48) 4500 (60)

Matching the PV Array to the Maximum Voltage Specifications of the Charge Controller Does the array match the MPPT controller? Assume the voltage co-efficient is 0.07V/°C.

If the minimum temperature is 20°C this is 5°C below the STC temperature of 25°C. Therefore the effective variation in voltage is: 5 x 0.07 = 0.35V

So the maximum open circuit voltage of the module = 22.1V + 0.35V =22.45V

Maximum number of modules that you can have in series = 150V (Maximum voltage for Flexmax 60)  22.45V = 6.68 this is rounded down to 6.

Some MPPT controllers might allow that the minimum array nominal voltage is that of the battery bank. However the MPPT will work better when the minimum nominal array voltage is higher than the nominal voltage of the battery. The Outback range of MPPT’s requires that the minimum nominal array voltage is 12 V greater than the battery voltage as shown in table below.

Minimum Nominal Array Voltages (Outback MPPT’s) Nominal Battery Voltage Recommended Minimum Nominal Array voltage 12V 24V 24V 36V 48V 60V

So the MPPT will allow between 3 and 6 modules in a string. The actual number of modules required was 8.68. If we round down to 8 (since an oversize factor of 10% and also worst month for PSH was used) then the array would be 2 parallel strings of 4 modules in series. If we round up to 9 then the solution could be 3 parallel strings with 3 modules in series.

Inverter Selection Which inverter to select based on the continuous and surge ratings?

From the load (energy) assessment on page a selected inverter must be capable of supplying 250VA continuous with a surge capability of 625VA.