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The watt (symbol: W) is the SI derived unit for power. It is equivalent to one joule per second (1 J/s), or in electrical units, one volt ampere (1 V·A).
It is the rate in joules per second at which energy is being converted, used, or dissipated.
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However, the V·A rating is only equivalent to watts when it applies to devices that absorb all the energy, such as electric heating coils or incandescent light bulbs. With e.g. computer power supplies, the actual watt rating is only 60% to 70% of the V·A rating.
The unit watt is named after James Watt for his contributions to the development of the steam engine, and was adopted by the Second Congress of the British Association for the Advancement of Science in 1889 and by the 11th Conférence Générale des Poids et Mesures in 1960.
A unit of power multiplied by a unit of time is a common format for expressing energy. For example, a kilowatt hour, the amount of energy expended by a one kilowatt device over the course of one hour, is 3.6 megajoules. A megawatt day (MWd or MW·d) is equal to 86.4 GJ. These units are often used in the context of power plants and home energy bills.
For the use of watts as a measurement in broadcasting, see effective radiated power and nominal power for a full discussion.
SI electricity units
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