Physical units are measures of distances, areas, volumes, heights, weights, mass, force, and energy. Different types of energy are measured by different physical units:

• Barrels or gallons for liquid petroleum fuels (such as gasoline, diesel fuel, and jet fuel) and biofuels (ethanol and biodiesel)
• Cubic feet for natural gas
• Tons for coal
• Kilowatthours for electricity

To compare fuels with each other, we need to convert their measurements to the same units.

Units for comparing energy

Some popular units for comparing energy include British thermal units (Btu), barrels of oil equivalent, metric tons of oil equivalent, metric tons of coal equivalent, and terajoules.

In the United States, Btu, a measure of heat energy, is the most commonly used unit for comparing energy sources or fuels. Because energy used in different countries comes from different places, Btu content of fuels varies slightly from country to country.

The Btu content of each fuel provided below (except for crude oil) is the average heat content for fuels consumed in the United States in 2017.

Btu content of common energy units

• 1 barrel (42 gallons) of U.S. produced crude oil = 5,722,000 Btu
• 1 gallon of finished motor gasoline (containing about 10% fuel ethanol by volume) = 120,429 Btu
• 1 gallon of diesel fuel (distillate fuel with less than 15 parts per million sulfur content) = 137,381 Btu
• 1 gallon of heating oil (distillate fuel with 15 to 500 parts per million sulfur content) = 138,500 Btu
• 1 barrel of residual fuel oil = 6,287,000 Btu
• 1 cubic foot of natural gas = 1,037 Btu
• 1 gallon of propane = 91,333 Btu
• 1 short ton (2,000 pounds) of coal (consumed by the electric power sector) = 19,015,000 Btu
• 1 kilowatthour of electricity = 3,412 Btu

Examples of converting different energy sources to Btu

Example 1:

You have a natural gas furnace in your home that used 81,300 cubic feet of natural gas for heating last winter. Your neighbor has a furnace that burns heating oil that used 584 gallons of heating oil last winter. You can convert the natural gas and heating oil consumption data into Btu to determine which home used more energy for heating.

Result: You used more energy to heat your home.

Example 2:

You and your neighbor want to compare the price of the fuels for heating your homes on an equal basis. You can compare the fuel prices in dollars per million Btu by dividing the price per unit of the fuels by the Btu content of the fuels in million Btu per unit.

Result: The price per million Btu for natural gas is about half the price of heating oil per million Btu.