Thenation's five largest fossil fuel generators burned almost 52% more gas in 2015than in 2011, according to an analysis of the period by SNL Energy.

Whilethese five companies still use more coal than gas, their coal burn in 2015 wasalmost 15% below 2011levels. The difference between gas and coal use has been narrowing since 2011.Oil burn, meanwhile, was almost 26% higher than 2011 levels, but less than thehigh it reached in 2014.

Thegenerators — NRG Energy Inc.(including NRG Yield Inc.),Duke Energy Corp.,Southern Co.,American Electric Power Co. Inc.and NextEra Energy Inc.— collectively own almost 179,000 MW of operating fossil fuel-fired capacity.

Forthe units owned by the five companies, gas-fired net generation overtookcoal-fired net generation for the first time in 2015, with 312,989 GWh comparedto coal's 312,138 GWh. Gas net generation accountedfor 50% of total fossil fuel-fired net generation for the five companies duringthe year, up 15 percentage points since 2011 and increasing over the five-yearperiod. NRG Energy and NRG Yield combined own the most fossil fuel-firedoperating capacity in the U.S., at about 45,500 MW. The two related generatorsgot a majority of fossil fuel-fired net generation from coal — 64% compared to34% of net generation from gas — although NRG's gas generation increased itsshare by 11 percentage points over the five-year period. NRGexpanded its fleet with its acquisition of GenOn Energy Inc. in 2012. The twocompanies own almost 23,000 MW of gas-fired capacity and almost 15,000 MW ofcoal-fired capacity. Oil-fired capacity accounts for the rest, at about 7,800MW.

Duke Energy saw a large swing in its generation mix over thefive-year period. In 2011, only 15% of its fossil fuel-fired net generation wasderived from gas-fired assets, but in 2015, gas accounted for 45% of thecompany's fossil fuel-fired generation. Duke added a large amount of gas-firedcapacity when it acquired ProgressEnergy Inc. in 2012. In all, Duke owns more than 40,000 MW offossil fuel-fired operating capacity, of which almost 20,000 MW is gas-fired,more than 18,000 MW is fired by coal and about 1,600 MW is fired by oil.

Southern Co. owns 37,600 MW of fossil fuel-fired operatingcapacity: about 21,000 MW of gas, 16,600 MW of coal and 1,000 MW of oil.Southern saw a flip of its gas-fired net generation and coal-fired netgeneration during the five-year period. In 2011, 58% of its fossil fuelgeneration was from coal and 42% was from gas, but by 2015 those numbersswapped, with 58% from gas and 42% from coal. Oil-fired generation contributedless than 1% to Southern's fossil fuel-fired generation every year since 2011.

AEP has the most coal-dominated generating fleet among thefive generators, with 68% of its fossil fuel-fired operating capacityattributed to coal, or about 20,000 MW of its total of just more than 29,000MW. While the percentage of its fossil fuel net generation mix attributable tocoal in 2015 was down 8 points from 2011, coal still made up 80% of AEP's totalfossil fuel net generation for the year. The company owns more than 9,000 MW ofgas-fired capacity but only 21 MW of oil-fired capacity.

In all, NextEra Energy owns about 26,600 MW of fossil fuelcapacity, with gas accounting for nearly 79% of this. NextEra's coal-firedcapacity was the lowest among the top five fossil fuel generators, at only 4%,while oil contributed 17% of NextEra's fossil fuel-derived capacity. Over the2011-2015 period, the company's gas generation increased to 92% of the totalfossil fuel generation, while coal and oil generation both declined from theiralready-low levels.