What is a British thermal unit?
A British thermal unit (Btu) is a measure of the heat content of fuels or energy sources. It is the quantity of heat required to raise the temperature of one pound of liquid water by 1 degree Fahrenheit at the temperature that water has its greatest density (approximately 39 degrees Fahrenheit).
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A single Btu is insignificant in terms of the amount of energy a single household or an entire country uses. In 2017, the United States used about 97.7 quadrillion Btu of energy. Written out, 1 quadrillion is a 1 followed by 15 zeros: 1,000,000,000,000,000.
Why use British thermal units?
Energy or heat content can be used to compare energy sources or fuels on an equal basis. Fuels can be converted from physical units of measure (such as weight or volume) to a common unit of measurement of the energy or heat content of each fuel. The U.S. Energy Information Administration (EIA) uses British thermal units as a unit of energy content.
EIA collects data on the physical amounts (volume or weight) of energy sources produced, imported, exported, and consumed. EIA converts those amounts into Btu equivalents to compare the sources on an equal basis.
For example, the physical volumes of fossil fuels consumed in the United States in 2017 and Btu equivalents were
- Petroleum–7.26 billion barrels–36.24 quadrillion Btu
- Natural gas–27.09 trillion cubic feet–28.03 quadrillion Btu
- Coal–716.96 million short tons–13.86 quadrillion Btu
Sample Btu conversion factors
| Energy source/fuel | Physical units and Btu (averages,¹ 2015) |
|---|---|
| Electricity | 1 kilowatthour = 3,412 Btu |
| Natural gas | 1 cubic foot = 1,037 Btu 1 therm = 100,000 Btu |
| Motor gasoline | 1 gallon = 120,429 Btu² |
| Diesel fuel | 1 gallon = 137,381 Btu |
| Heating oil | 1 gallon = 138,500 Btu |
| Propane | 1 gallon = 91,333 Btu |
| Wood | 1 cord = 20,000,000 Btu³ |
¹ Weighted averages for energy sources/fuels as consumed by end-use sectors.
² Gasoline sold at retail in the United States, with about 10% ethanol content by volume.
³ This conversion is an estimate. A cord of wood is a volume unit and does not take wood density or moisture content into account. Wood heat content varies significantly with moisture content.
The Btu conversion factors above are approximations. EIA provides more heat contents for fuels and electricity in the appendices of the Monthly Energy Review.