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watt

For the meaning of any metric prefix, go here.

Convert between watts and kilowatts and other major units of power.

The unit of power in SI. Symbol, W (no period).

Power is a rate, the rate at which energy is expended or work done. The watt is defined as “the power which in one second gives rise to energy of 1 joule” (Resolution 2 of the CIPM, 1946, ratified by the Ninth CGPM in 1948). In mechanical terms, a power of 1 watt can, in 1 second, move a mass of 1 kilogram through a distance of 1 meter with such force that the kilogram's velocity at the end of the meter will be 1 meter per second greater than it was at the beginning. In an electric circuit, 1 watt is a current of 1 ampere at a pressure of 1 volt.

The watt's dimensions are (L²M)/T³ — joules per second, or in terms of base units,

$the fraction meters squared time kilograms over seconds cubed$

The watt was introduced at the Second International Congress of Electricians (Paris, 1889), where it was defined as 10⁷ ergs per second. (The same conference introduced the joule, defining it as 10⁷ ergs.)

picture of James Watt

The watt is named for James Watt (1736-1819), inventor of the condensing steam engine.

sources

For an excellent history of the development of the electric and magnetic units up to 1913, see U. S. Bureau of Standards Circular 60.