Energies 2014, 7, 7955-7993; doi:10.3390/en7127955 OPEN ACCESS energies ISSN 1996-1073 www.mdpi.com/journal/energies Article U.S. Energy Transitions 1780–2010 Peter A. O’Connor * and Cutler J. Cleveland Department of Earth and Environment, Boston University, 685 Commonwealth Avenue, Boston, MA 02215, USA; E-Mail:
[email protected] * Author to whom correspondence should be addressed; E-Mail:
[email protected]; Tel.: +1-703-400-3464. External Editor: David I. Stern Received: 15 July 2014; in revised form: 4 November 2014 / Accepted: 5 November 2014 / Published: 27 November 2014 Abstract: Economic and social factors compel large-scale changes in energy systems. An ongoing transition in the United States is driven by environmental concerns, changing patterns of energy end-use, constraints on petroleum supply. Analysis of prior transitions shows that energy intensity in the U.S. from 1820 to 2010 features a declining trend when traditional energy is included, in contrast to the “inverted U-curve” seen when only commercial energy is considered. This analysis quantifies use of human and animal muscle power, wind and water power, biomass, harvested ice, fossil fuels, and nuclear power, with some consumption series extending back to 1780. The analysis reaffirms the importance of innovation in energy conversion technologies in energy transitions. An increase in energy intensity in the early 20th century is explained by diminishing returns to pre-electric manufacturing systems, which produced a transformation in manufacturing. In comparison to similar studies for other countries, the U.S. has generally higher energy intensity. Keywords: energy transitions; economic history; energy intensity 1.