The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. A human climbing a flight of stairs is doing work at the rate of about 200 watts. A first class athlete can work at up to approximately 500 watts for 30 minutes. An automobile engine produces mechanical energy at a rate of 25,000 watts (approximately 30 horsepower) while cruising. A typical household incandescent light bulb uses electrical energy at a rate of 40 to 100 watts.





One watt is one joule (the SI unit of energy) per second, that is 1 newton metre per second. It may be visualized simply as the amount of energy expended by a single candle.




kg x m2

N x m

1W = 1


= 1


= 1






In electrical terms, it follows that:


1W = 1V x 1A



That is, if 1 volt of potential difference is applied to a resistive load, and a current of 1 ampere flows, then 1 watt of power is dissipated.





The 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.





The kilowatt (symbol: kW) is a unit for measuring power, equal to one thousand watts. A kilowatt is roughly equivalent to 1.34 horsepower.





The megawatt (symbol: MW) is equal to one million (106) watts.


Many things can sustain the transfer or consumption of energy on this scale; some of these events or entities include: lightning strikes, large electric motors, naval craft (such as aircraft carriers and submarines), engineering hardware, and some scientific research equipment (such as the supercollider and large lasers). A large residential or retail building may consume several megawatts in electric power and heating energy.


The productive capacity of electrical generators operated by utility companies is often measured in MW. Modern high-powered diesel-electric railroad locomotives typically have a peak power output of (3 to 5) MW, whereas a typical modern nuclear power plant produces a peak output on the order of 500 to 2000 MW.


According to the Oxford English Dictionary, the earliest citing for "megawatt" is a reference in the 1900 Webster's International Dictionary of English Language. The OED also says "megawatt" appeared in a 28 November 1847, article in Science (506:2).



Watts electrical and thermal


Watt electrical (abbreviation: We) is a term that refers to power produced as electricity. SI prefixes can be used, for example megawatt electrical (MWe) and gigawatt electrical (GWe). Watt thermal (abbreviation: Wt or Wth) refers to thermal power produced; again SI prefixes can be used, for example megawatt thermal (MWt) and gigawatt thermal (GWt). These terms are used to disambiguate the electric output of a thermal power station versus the (larger) thermal output. For example, the Embalse nuclear power plant in Argentina uses a fission reactor to generate 2109 MWt of heat, which creates steam to drive a turbine, which generates 648 MWe of electricity.


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