To compare or aggregate energy consumption across different energy sources like oil, natural gas, and electricity, we must use a common unit of measure. This is similar to calculating your food energy intake by adding up the calories in whatever you eat.
In American households we use several kinds of energy. It's difficult to add up or compare the total energy we use because each energy source comes in a different unit: gasoline is in gallons, electricity is in kilowatthours, and natural gas is in cubic feet. One way to add and compare different energy sources is to convert them all to one common unit of measure based on their energy content.
The Energy Information Administration uses the British thermal unit (Btu) as its common energy unit.
A Btu is approximately equal to the amount of energy that comes from burning one wooden kitchen match.1 A Btu isn't an everyday term to most people, but you might see it on your energy bill or in a newspaper article.
Because a Btu is such a small unit of energy, there are tens of thousands of Btu even in one gallon of gasoline. The table at left shows how to convert different energy sources into Btu units.
Btus are to energy as calories are to food.
You probably already have experienced converting from physical units to energy units. When calculating the total "amount" of food you eat, you might look up how many calories are in each item and then add up the calories. You can't add a hamburger and a soft drink without the conversion. So you can see that calories are a common unit for measuring the energy content of food.
Let's say you consume a typical fast-food meal of:
Hamburger 1/4 pound 300 calories
French fries 50 fries 400 calories
Soft drink 16 ounce 200 calories
Total 900 calories
If you ate the items listed, you would have consumed 900 calories. Just as calories are a useful measure to help you compare different food items, Btus are useful for making energy comparisons.
If you want to calculate the total amount of energy you use, the process is similar. You can take the gallons of gasoline, the heating fuel amounts, and the electricity to run your home appliances and convert them all to Btu equivalents, using the conversion rates in the table. Then you can add up the different pieces to get a total amount in common units.
One wrinkle is that electricity is an energy carrier, or secondary fuel source, rather than a primary fuel. There are significant losses in the conversion of primary fuels to electricity and in transmission and distribution of electricity to where it is used. For example, in 2007 the average coal-fired plant used 10,114 Btu of coal to generate one kilowatt hour (=3,412 Btu) of electricity. For each kilowatt hour (kWh) of electricity used, about 220 Btu (0.065 kWh) are used in the power lines from the power plant to your house. If your focus is on primary energy use (such as coal, natural gas, or oil), you should start your calculation with the energy used to make and deliver electricity instead of the energy in the electricity itself.
Most people are interested in saving energy these days, and you can use Btu equivalents to help you compare the different levels of savings resulting from taking different actions or making lifestyle changes. Which do you think uses more energy in a year: gasoline in the average car or electricity in the average home? It's easy to find the answer if you make some assumptions about what is average and then convert the numbers to Btu. See the answers below.
Total energy used by one average U.S. car per year…
Driven 12,300 miles per year at 22.5 miles per gallon equals 547 gallons of gasoline at 124,000 Btu per gallon = 67.8 million Btu
Total electricity used by one average U.S. household in 2008…
Total U.S. residential electricity used is 1,379 billion kWh divided by 115 million households at 3,412 Btu per kWh = 40.9 million Btu
Total primary energy used to provide the electricity used by one average U.S. household per year…
Total energy input to electricity production is 40.7 quadrillion Btu times the residential share of electricity use of 37% divided by 115 million households = 131.0 million Btu
It's interesting to see in these comparisons that residential use of energy for electricity appears to be lower than that for an average car when you use the consumption Btu value of 3,412 Btu per kWh for electricity. But if you count all the primary energy used to generate and deliver the electricity, average residential use of energy for electricity is actually much higher than it is for a car.
Here's another way to compare energy use. Suppose you hear about a new energy efficiency plan that will save 1,000 trillion Btu per year, which is about 1% of total U.S. annual energy use. A trillion of anything is a big number to visualize. Sometimes it's easier to appreciate how much energy that is by thinking in terms of cars or houses, just like it's easier to think of calories as hamburgers and fries, not the energy units themselves.
You could divide the energy used by one car (67.8 million Btu) into 1,000 trillion Btu to find that the energy savings in the same plan shown above is equal to about 14.8 million cars taken off the roads. These averages give you a way to envision and understand the magnitude of the energy issues and solutions being considered for our energy future.