From 2006 to 2015, annual average heat rates of natural gas-fired electricity generators decreased 7% as heat rates of coal-fired electricity generators remained stable, increasing only 1%. Heat rates are calculated based on the amount of energy (measured in British thermal units) reported to the U.S. Energy Information Administration (EIA) that was used to generate a unit of electricity. Lower heat rates indicate more efficient generation, because less fuel is needed per kilowatt hour.
The small rise in the average operating heat rate for coal-fired generation may be attributed to emissions controls. Emissions-control equipment was installed on almost 205 gigawatts (GW) of coal capacity from 2006 to 2015, or about 73% of the coal-fired generator fleet that was operating in 2016. These emissions-control measures often require more on-site usage of electricity, which involves consuming fuel but not necessarily producing electricity output.