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The watt-hour (symbol W·h) is a unit of energy. It is commonly used in the form of the kilowatt-hour, which is 1,000 watt-hours. It is not an SI unit, despite being based on the watt. The SI unit of energy is the joule (J), equal to one watt-second. Even if the watt-hour were not redundant, it could not be an SI unit, as the hour is not. It is, however, a commonly used unit, especially for measuring electric energy.
Definition One watt-hour is the amount of electrical energy equivalent to a one-watt load drawing power for one hour. This is equivalent to 3,600 joules, the joule being the SI unit of energy. Thus a kilowatt-hour is 3,600,000 joules or 3.6 megajoules. For example, a 60-watt light bulb uses 60 watt-hours of energy every hour. Similarly, a 100-watt light bulb uses 100 watt-hours in an hour. The kilowatt-hour is usually used by U.S. power companies to measure power usage, so a rate of US$0.10 per kilowatt-hour will cost US$0.10 to run a 1,000-watt light bulb for one hour. Similarly UK power companies define 1 unit of electricity as 1 kilowatt-hour, which costs anywhere between 4 and 20 UK pence depending upon tariff. Multiples Kilo-, mega-, giga-, and tera- are the most-used prefixes. Conversions Explanation The watt-hour is derived from the multiplication of the SI unit of power (watt) and a non-SI unit of time (hour). The kilowatt-hour is commonly used for electrical and natural gas energy. Many electric utility companies use the kilowatt-hour for billing. This is a convenient unit because the energy usage of a typical home in one month is several hundred kilowatt-hours. Megawatt-hours are used for metering of larger amounts of electrical energy. For example, a power plant's daily output is likely to be measured in megawatt-hours. The Board of Trade Unit or B.O.T.U. is an obsolete UK synonym for kilowatt-hour. The term derives from the name of the Board of Trade that regulated the electricity industry. The B.O.T.U. should not be confused with the British thermal unit or BTU, which is a much smaller quantity of thermal energy. See also | ||||||||
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