The U.S. electricity sector has seen tremendous growth in the past 60 years. From 1949 to 2009, U.S. electricity consumption increased by a factor of 13. To meet this rising demand, the U.S has built vast amounts of new electricity generating infrastructure. The total U.S. installed capacity in 2009 was 998 GW, compared with just 65 GW in 1949.
U.S. electricity generating capacity has gone through several build-out phases. From 1949 to 1983, the majority of additions to U.S. electricity generating capacity were coal-fired power plants. For a brief period of four years, nuclear power was the major contributor to annual capacity growth, but was quickly overtaken by natural gas, which has dominated growth since. Almost all of the added U.S. capacity between 1990 and 2005 was from natural gas power plants. Only in the past few years has windpower become a major contributor to nationwide capacity growth.
U.S. electricity demand rose steadily from 1949 to 2007, reaching a peak at just over 4,000 TWh/y. In the past 15 years, this growth has been largely driven by the residential and commercial sectors, while industrial electricity demand has actually been declining slightly. But since 2007, nationwide electricity demand has been declining, and this trend is expected to continue.
Not all generating resources have the same capacity factor—the amount of energy they actually generated in a year divided by the amount they would have generated running at full nameplate capacity all year long. The differences in capacity factor between resources are evident in comparing these charts of installed capacities and actual annual generations. Looking at 2009 numbers, for example, coal makes up 31% of U.S. installed capacity, but accounts for 46% of annual generation. Natural gas, on the other hand, accounts for 39% of installed capacity but only 22% of generation. These differences reflect the fact that coal-fired plants are traditionally operated as close to full capacity as possible around-the-clock, while natural gas plants, having higher operating costs and greater flexibility, are often used as peakers, ramping their capacity up and down throughout the day to meet peak demand. (However, starting in late 2008, combined-cycle gas plants often became cheaper to run than coal plants, so they were run more and coal plants less.)
For more information on the age of U.S. power plants currently in operation (of all fuel types), see the Energy Information Administration’s website on this topic:
A note on capacity data: EIA data include separate U.S. coal, oil, and natural gas capacities from 1989 to the present, but only the gross fossil capacity from 1949-1988. RMI estimated the installed capacity for each independent fossil resource over this period using annual electricity generation by resource type and estimated capacity factors.
U.S. Energy Information Administration. 2010. “Annual Electric Generator data.” Form EIA-860 Data Files. link
U.S. Energy Information Administration. 2010. Electric Power Annual 2009. Washington, D.C.: U.S. Department of Energy, November 23. link