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What Is Energy?

Energy Units Basics

Energy Ant

How do we compare different types of fuels? One practical way is to convert the physical units of the fuels into British thermal units or “Btu.”
A Btu is a precise measure of the heat content of fuels.

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How big is a barrel?

A barrel is a unit of volume or weight that is different depending on who uses the term and what it contains...

For example:

1 barrel (b) of petroleum or related products = 42 gallons

1 barrel of Portland cement = 376 pounds

1 barrel of flour = 196 pounds

1 barrel of pork or fish = 200 pounds

1 barrel of (U.S.) dry measure = 3.29122 bushels or 4.2104 cubic feet

A barrel may be called a "drum," but a drum usually holds 55 gallons.

A barrel big enough to carry a man over Niagara Falls.

Physical units reflect 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 different fuels, we need to convert the 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 fuels. Because energy used in different countries comes from different places, Btu content of fuels varies slightly from country to country.

Btu content of each fuel provided below is the average heat content for fuels consumed in the United States in 2015.

Btu content of common energy units

  • 1 barrel (42 gallons) of crude oil = 5,729,000 Btu (for U.S.-produced crude oil)
  • 1 gallon of gasoline = 120,405 Btu
  • 1 gallon of diesel fuel = 137,381 Btu (distillate fuel with less than 15 parts per million sulfur content)
  • 1 gallon of heating oil = 138,500 Btu (distillate fuel with 15 to 500 parts per million sulfur content)
  • 1 barrel of residual fuel oil = 6,287,000 Btu
  • 1 cubic foot of natural gas = 1,032 Btu
  • 1 gallon of propane = 91,333 Btu
  • 1 short ton (2,000 pounds) of coal = 19,882,000 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.

Natural gas: 81,300 cubic feet (your home)


1,032 Btu per cubic foot

= 83,901,600 Btu

Heating oil: 584 gallons (neighbor's home)


138,500 Btu per gallon

= 80,884,000 Btu

Result: You used more energy to heat your home.

Example 2:

You need a new furnace for your home, and you are comparing heating systems that use natural gas with systems that use heating oil. One factor to consider is the cost of the fuels. You can compare the price of the fuels on an equal basis by dividing the price per unit of the fuels by the Btu content of the fuels in million Btu per unit to get the price in dollars per million Btu.

Natural gas:

$10.40 per thousand cubic feet


1.032 million Btu per thousand cubic feet 

= $10.08 per million Btu

Heating oil:

$2.70 per gallon


0.1385 million Btu per gallon 

= $19.49 per million Btu

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