The US Energy Information Administration (EIA) reported that the natural gas-fired electricity conversion efficiency is growing while the coal-fired plants efficiency remained stable. The average annual heat rate of gas-fired power plants decreased by 7% between 2006 and 2015, while the coal-fired power plants heat rate remained stable over the same period, increasing by only 1%. As a result, the average heat rate for natural gas-fired generation was about 25% lower than the average heat rate for coal-fired generation in 2015.
The heat rate is the amount of energy (measured in British thermal units - Btu) used to produce a unit of electrical energy. Lower heat rates indicate more efficient generation processes because less fuel is needed per kWh.
According to the EIA, this discrepancy can be explained by emissions controls equipment, which has been set up on almost 205 GW of coal capacity over the 2006-2015 period (73% of the operational coal-fired generating fleet as of 2016). These measures involve more on-site power usage and require fuel consumption while not necessarily increasing the electricity output. Besides, combined-cycle gas-fired plants (CCGT) are more efficient than simple cycle power stations, with respective average heat rates amounting 7,340 Btu/kWh and 9,788 Btu/kWh, while coal units have a weighted average design heat rate of 10,343 Btu/kWh.