Measuring Energy

Energy exists in many forms, so there are many ways to quantify it. Two of the most widely used for general purposes are the British Thermal Unit (BTU), which is a measure of energy content, and the watt, which is a measure of power, or how fast energy is used.

One BTU is the amount of energy needed to raise a pound of water by one degree Fahrenheit. That’s not a very large amount. One cubic foot of natural gas contains around 1,000 BTUs. A gallon of gasoline is about 124,000 BTUs, and a ton of coal represents about 20 million BTUs. Enormous quantities, such as total U.S. energy consumption in a year, are expressed in “quads.” One quad is a quadrillion—that is, a million billion, or 1015—BTUs. America consumed about 100 quads in 2006.

One watt of power is equal to one ampere (a measure of electric current) moving at one volt (a measure of electrical force). Again, this is a fairly small unit. U.S. household electricity is provided at 120 volts. So a 60-watt lightbulb needs half an ampere of current to light up. For larger quantities, watts are usually expressed in multiples of a thousand (kilowatt), million (megawatt), or billion (gigawatt). A big coal, natural gas, or nuclear electrical plant can produce hundreds of megawatts; some of the largest generate one or more gigawatts. A typical wind turbine has a one megawatt rating, and the largest are now four megawatts when turning. An average U.S. household consumes electricity at the rate of a little more than one kilowatt, for an annual total of about 10,000 kilowatt-hours (kilowatt-hours equal power multiplied by time).

cars charge at a rate roughly a thousand times slower than the rate of refueling with gasoline, meaning overnight charging is required to store enough energy for a day’s worth of driving. For most Americans in the fast-paced 21st century, that’s an unacceptably long time span.


Energy trade-offs and decisions permeate society, directly affecting everyday quality of life in many ways. Some effects may be most noticeable at home—or at least in household energy bills due to the rising costs of heating oil and natural gas. Residential energy use accounts for 21% of total U.S. consumption, and about one-third of that goes into space heating, with the rest devoted, in decreasing proportions, to appliances, water heating, and air-conditioning. So our personal preferences

Percentage of energy consumed by each economic sector in the United States in 2006.*

Percentage of energy consumed by each economic sector in the United States in 2006.*

* Percentages do not sum to 100% due to independent rounding.

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