At the end of 2010, natural gas-fired generators constituted 39% of the nation's total electric generation capacity of 1,042 gigawatts (GW). Nearly 237 GW of natural gas-fired generation capacity was added between 2000 and 2010, representing 81% of total generation capacity additions over that period.
About 65% of the existing natural gas capacity added after 1980 is combined-cycle units. As a whole, average utilization of the nation's fleet of natural gas combined-cycle gas has been increasing over the past six years. Other recent natural gas capacity additions are primarily combustion turbines, typically used as peaking units. Most of the natural gas-fired capacity additions from the 1950s through the 1970s were steam turbines—some of these were dual-fired units capable of burning natural gas and oil. However, air pollution restrictions and the current price differences between gas and oil mean that these units are primarily burning gas. Although these gas-fired steam units were built as baseload generators, they now operate as load-following or peaking generators, that are dispatched only when higher efficiency, combined-cycle units are fully utilized or are otherwise unavailable to serve load requirements. (U.S. Energy Information Administartion-EIA)