How can we compare or add up our energy consumption?

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 is typically measured in a different unit: gasoline is usually measured in gallons, electricity in kilowatthours, and natural gas in cubic feet. One way to add and compare different energy sources is to convert them all to a common unit of measure based on their energy content.

One Btu is the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit. It is approximately equal to the amount of energy that comes from burning one wooden kitchen match. A Btu isn't an everyday term to most people, but you might see it on your energy bill or in a news article.

Because a Btu is such a small unit of energy, there are tens of thousands of Btus in even one gallon of gasoline. The table to the right shows how to convert different energy sources into Btus.

You probably already have experienced converting 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:

If you ate the items listed above, 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 consumed by your car, the amount of natural gas and other fuels that heat your home, and the kilowatthours of electricity to run your lights and 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 source. There are significant losses in the conversion of primary fuels to electricity and in the transmission and distribution of electricity to the consumer.

For example, in 2011 the average coal-fired plant used 10,400 Btu of coal to generate one kilowatt hour (=3,412 Btu) of electricity. (Of course, there are regional differences in the primary energy used to generate electricity, and not all generation comes from thermal sources with the associated thermal energy losses.) In addition, another 7-8% of the electricity is used up when it is transmitted and distributed 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 compare the different levels of savings resulting from taking different actions or making various 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 average usage and then convert the numbers to Btu. See the answers below.

It's interesting to see in these comparisons that residential use of energy for electricity appears to be lower than that for an average vehicle 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 single vehicle. However, nearly 60% of households have two or more vehicles, making the average household use of energy for electricity about the same as it is for two passenger vehicles.

Here's another way to compare energy use. Suppose you hear about a new energy efficiency proposal that will save 1,000 trillion Btu per year, which is about 1% of total U.S. annual energy use. A trillion is a big number to visualize. However, sometimes it's easier to appreciate how much energy is represented by thinking in terms of cars or houses, just like it's easier to think of calories as hamburgers and fries, not the calories themselves.

You could divide the energy used by one car/vehicle (66 million Btu) into 1,000 trillion Btu to find that the energy savings in the same proposal described above is equal to taking approximately 15.2 million vehicles off the roads. These averages provide a way to visualize and understand the magnitude of the energy issues and solutions being considered.

The average passenger car/vehicle (including light trucks, vans, and sport utility vehicles) in the United States uses about 66 million Btu per year, which sounds like a big number for just one vehicle. But total energy use for cars, light trucks, vans, and sport utility vehicles in 2010 was about 15 quadrillion Btu, which is 15 with 15 zeros added on to it. That was equivalent to about 16% of total U.S. energy consumption in 2010.

Energy sources are expressed in different units, but their energy content can be compared using the British thermal unit (Btu)

Conversion Table of Common Energy Sources to Btu Energy SourcePhysical Units and Btu Equivalents1 kilowatthour (kWh) = 3,412 Btu (but on average, it takes about 3 times the Btu of primary energy to generate the electricity)1 cubic foot (ft3) = 1,022 Btu
1 cubic foot = 0.01 therms
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