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MkWh = make a wish and mKWH = Million Kilo-watt hour mkwh.com The watt-hour (symbol W·h or Wh) is a unit of energy. It is most commonly used on household electricity meters in the form of the kilowatt-hour (kW·h or kWh), which is 1,000 watt-hours. It is not an SI unit, despite being based on the watt, as the hour is not an SI unit. The SI unit of energy is the joule (J), equal to one watt-second. It is, however, a commonly used unit, especially for measuring electric energy. Definition One watt-hour is the amount of (usually electrical) energy expended by a one-watt load (e.g., light bulb) drawing power for one hour. Laymen and utilities tend to use watt-hours to measure energy rather than joules, for reasons of convenience and intuition: For example, a light bulb draws power (units of watts) over a certain amount of time, resulting in a net amount of used energy: a watt has units of energy-per-time, and an hour is a convenient unit for measuring time, so when multiplied together they produce a unit of energy called the watt-hour. 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. In addition, the typical consumer can readily conceptualize the notion of "using a kilowatt for one hour." 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. Power companies produce energy — a good — which is often purchased by the customer in units of kilowatt-hours. Consider a setup with two 50 W light bulbs (100 W total) left on for 10 hours per day. The setup will consume 1 kilowatt-hour per day. If a power company charges $0.10/kW·h, then those two light bulbs will cost $0.70 over the course of a week. (See unit juggling for more information.) 1 watt-hour 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.) Some sources mistakenly refer to watt-hours as "power". A similar confusion can arise when describing daily energy use. For example, a solar cell array might have a peak power output of 100 watts, but in order to give an indication of its usable output as a function of time-varying conditions (such as the apparent daily solar motion, or dust collection on the surface), its typical output might be described as “1200 watt-hours per day.” Different writers may disagree as to whether this is a measure of power or energy usage. Another derived unit that is sometimes used for household purposes is the kWh/yr, usually considered in annual energy consumption calculations, but with the dimensions of power, with 1 kWh/yr = 0.114 W. Note that this unit uses three units of time in one unit, namely second, hour and year, of which only the first is an SI unit. The Board of Trade Unit or B.O.T.U. is an obsolete U.K. 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.
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