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EIA uses the heat content of fossil fuels to compare and aggregate energy sources

The four end-use sectors in the United States—residential, commercial, industrial, and transportation—consume a mix of fossil fuels (petroleum, natural gas, and coal), as well as renewable energy sources and electricity, to meet their energy needs. To compare and aggregate data from these different energy sources, the U.S. Energy Information Administration (EIA) measures consumption in a common energy unit, British thermal units (Btu), which describes the heat content of the fuels. EIA uses thermal conversion factors to convert the physical units that are generally measured and reported (such as barrels of petroleum or cubic feet of natural gas) into heat content values.