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What is energy?

Energy units basics

How do we compare different types of fuels? One practical way is to convert the physical units of fuels, such as weight or volume, into British thermal units or "Btu." A Btu is a precise measure of the heat content of fuels.

Physical units are measures of distances, areas, volumes, heights, weights, mass, force, and energy. We use different physical units to measure dfferent types of energy or fuels:

  • 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 (a short ton equals 2,000 pounds; a metric ton equals about 2,205 pounds)
  • 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 common 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.

Btu content of common energy units (preliminary estimates for 20211):

  • 1 barrel (42 gallons) of crude oil produced in the United States = 5,691,000 Btu
  • 1 gallon of finished motor gasoline (containing about 10% fuel ethanol by volume) = 120,238 Btu
  • 1 gallon of diesel fuel or heating oil (with sulfur content less than 15 parts per million) = 137,381 Btu
  • 1 gallon of heating oil (with sulfur content at 15 to 500 parts per million) = 138,500 Btu
  • 1 barrel of residual fuel oil = 6,287,000 Btu
  • 1 cubic foot of natural gas = 1,039 Btu
  • 1 gallon of propane = 91,452 Btu
  • 1 short ton (2,000 pounds) of coal (consumed by the electric power sector) = 18,934,000 Btu
  • 1 kilowatthour of electricity = 3,412 Btu

Examples of converting energy sources in different physical units to Btu

Example 1:

You have a natural gas furnace in your home that used 67,000 cubic feet of natural gas for heating last winter. Your neighbor has a furnace that burns heating oil that used 500 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.

  • Natural gas
  • 67,000 cubic feet (your home)
  • x
  • 1,039 Btu per cubic foot
  • =
  • 69,613,000 Btu
  • Heating oil
  • 500 gallons (neighbor's home)
  • x
  • 137,381 Btu per gallon
  • =
  • 68,690,476 Btu

Result: You used more energy to heat your home. (Note that many factors affect the amount of energy a household actually uses for heating.)

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.

  • Natural gas
  • $13.52 per thousand cubic feet
  • ÷
  • 1.0309 million Btu per thousand cubic feet
  • =
  • $13.01 per million Btu
  • Heating oil
  • $3.76 per gallon
  • ÷
  • 0.137381 million Btu per gallon
  • =
  • $27.36 per million Btu

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

1Source: Monthly Energy Review, June 2022