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The watt (symbol: W) is the SI derived unit of power, equal to one joule of energy per second. It measures a rate of energy conversion.

A human climbing a flight of stairs is doing work at a rate of about 200 watts. A typical automobile engine produces mechanical energy at a rate of 25,000 watts (approximately 33.5 horsepower) while cruising. A typical household incandescent light bulb uses electrical energy at a rate of 25 to 100 watts, while compact fluorescent lights typically consume 5 to 30 watts.

One watt is the rate at which work is done when an object is moving at one meter per second against a force of one newton.

1~\rm{W} = 1~\dfrac{\rm{J}}{\rm{s}} = 1~\dfrac{\rm{kg} \cdot \rm{m^2}}{\rm{s^3}} = 1~\dfrac{\rm{N\cdot m}}{\rm{s}} 

By the definition of the units of ampere and volt, work is done at a rate of one watt when one ampere flows through a potential difference of one volt.[1]

1~\rm{W} = 1~\rm{V \cdot } 1~\rm{ A}


Origin and adoption as an SI unit

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 General Conference on Weights and Measures in 1960 as the unit of power incorporated in the International System of Units (or "SI").

This SI unit is named after James Watt. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (W). When an SI unit is spelled out in English, it should always begin with a lowercase letter (watt), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.

Based on The International System of Units, section 5.2.

Derived and qualified units for power distribution


The nanowatt is equal to one billionth of a watt. From a single star of magnitude +3.5 a square meter receives one nanowatt.


The microwatt (symbol:μW) is equal to one millionth (10-6) of a watt.


The milliwatt (symbol:mW) is equal to one thousandth (10-3) of a watt. A typical laser pointer might output 5 milliwatts.


The kilowatt (symbol: kW), equal to one thousand watts, is typically used to state the power output of engines and the power consumption of tools and machines. A kilowatt is approximately equivalent to 1.34 horsepower. An electric heater with one heating element might use 1 kilowatt. The average power consumption, over a year, of a household in the United States is about 8,900 kilowatt-hours[2].


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 supercolliders 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 U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW.[3]

The earliest citing for "megawatt" in the Oxford English Dictionary is a reference in the 1900 Webster's International Dictionary of English Language. The OED also says "megawatt" appeared in a 28 November 1947, article in Science (506:2).


The gigawatt (symbol: GW) is equal to one billion (short scale) (109) watts. This unit is sometimes used with large power plants or power grids.


The terawatt (symbol: TW) is equal to one trillion (short scale) (1012) watts. The total power used by humans worldwide (about 15 TW) is commonly measured in these units. The most powerful lasers from the mid 1960s to the mid 1990s produced power in terawatts, but only for nanoseconds.


The petawatt (symbol: PW) is equal to one quadrillion (short scale) (1015) watts and can be produced by the current generation of lasers for time-scales of the order of femtoseconds (10-15).

Electrical and thermal

In the electric power industry, megawatt electrical (abbreviation: MWe[4] or MWe[5]) is a term that refers to electric power, while megawatt thermal or thermal megawatt [6](abbreviations: MWt, MWth, MWt, or MWth) refers to thermal power produced. Other SI prefixes are sometimes used, for example gigawatt electrical (GWe). [7]

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. The difference is due to the inefficiency of steam-turbine generators and the limitations of the theoretical Carnot Cycle.

Confusion of watts and watt-hours

Power and energy are frequently confused in the general media. Power is the rate at which energy is used (or generated). A watt is one joule of energy per second. For example, if a 100 watt light bulb is turned on for one hour, the energy used is 100 watt-hours or 0.1 kilowatt-hour, or (60x60x100) 360,000 joules. This same quantity of energy would light a 40-watt bulb for 2.5 hours. A power station would be rated in watts, but its annual energy sales would be in watt-hours (or kilowatt-hours or megawatt-hours). A kilowatt-hour is the amount of energy equivalent to a steady power of 1 kilowatt running for 1 hour, or 3.6 megajoules.

Terms such as 'watts per hour', which are sometimes used in the media, are meaningless in practice (unless referring to change of power per hour).[8]

Often energy production or consumption is expressed as terawatt-hours per year. As one year contains about 8,765.82 hours, one terawatt-hour per year equals about 114,080 megawatts.

See also


  1. ^ "Amps, Volts, Watts, Ohms". http://ebtx.com/mech/ampvolt.htm. Retrieved on 17 April 2007. 
  2. ^ "The Physics Factbook". http://hypertextbook.com/facts/2003/BoiLu.shtml. Retrieved on 17 February 2009. 
  3. ^ Nuclear Regulatory Commission. (2007). 2007–2008 Information Digest. Retrieved on 27 January 2008. Appendix A.
  4. ^ Cleveland, C. J. 2007. "Watt". Encyclopedia of Earth.
  5. ^ How Many? A Dictionary of Units of Measurement
  6. ^ http://apps1.eere.energy.gov/news/news_detail.cfm/news_id=12362
  7. ^ 'Megawatt electrical' and 'megawatt thermal' are not SI units,Taylor 1995, Guide for the Use of the International System of Units (SI), NIST Special Publication SP811 The International Bureau of Weights and Measures states that unit symbols should not use subscripts to provide additional information about the quantity being measured, and regards these symbols as incorrect. International Bureau of Weights and Measures. (2006). The International System of Units (SI). 132.
  8. ^ Inverter Selection. (n.d.) Northern Arizona Wind and Sun. Retrieved 27 March 2009.

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