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).