Power is a rate of doing work. Frequently it is the rate at which one form of energy is transformed into another. A 100-watt light bulb, for example, is capable of transforming electric energy into light and heat at the rate of 100 joules per second.

Units of power are often confused with units of energy. A light bulb, for example, might be rated “100 watts.” Watts are a measure of power. If you burn the bulb for 200 hours, the electric bill for the quantity of energy you bought will read 20 kilowatt-hours. The kilowatt-hour is a measure of energy.

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Last revised: 13 July 2011.