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The 'watt' (symbol: 'W') is the SI derived unit of power, equal to one joule of energy 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 375 watts for 30 minutes before exhaustion.^[1] 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, while the energy-saving compact fluorescent lights which are replacing them use 8 to 20 watts to yield the same light output.

Contents

Definition

Origin and adoption as an SI unit

SI multiples

Derived and qualified units for power distribution

Kilowatt

Megawatt

Examples of other derived SI units

Watts electrical and thermal

Confusion of watts and watt-hours

See also

References

External links

Definition

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.
:$1\; mathrm\{W\}\; =\; 1\; dfrac\{mathrm\{J\}\}\{mathrm\{s\}\}\; =\; 1\; dfrac\{mathrm\{kg\}\; cdot\; mathrm\{m^2\}\}\{mathrm\{s^3\}\}\; =\; 1\; dfrac\{mathrm\{Ncdot\; m\}\}\{mathrm\{s\}\}.$
In electrical terms, it follows that:
:$1\; mathrm\{W\}\; =\; 1\; mathrm\{V\}\; cdot\; 1\; mathrm\{A\}.$
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.^[2]

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").

SI multiples

Derived and qualified units for power distribution

Kilowatt

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 roughly equivalent to 1.34 horsepower.

Megawatt

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).

Examples of other derived SI units

The power output of the Sun is 390 yottawatts. On the other hand, the signal produced by cosmological dark matter axions resonating in a microwave cavity experiment would be a few yoctowatts.

Watts electrical and thermal

'Watt electrical' (abbreviation: We) is a term that refers to power produced as electricity, while 'Watt thermal' (abbreviation: Wt or Wth) refers to thermal power produced. These terms and abbreviations are not SI units, but they are used with the same prefixes as SI, for example ''megawatt electrical'' (MWe), ''gigawatt electrical'' (GWe), ''megawatt thermal'' (MWt) and ''gigawatt thermal'' (GWt). These terms are used by engineers 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. The difference is heat lost to the surroundings.

Confusion of watts and watt-hours

Power and energy are frequently confused in the general media, for instance when a device is said to be rated at "100 watts per hour", which does not make any sense since a watt is a rate of doing work or using energy of 1 joule of energy per second. As a rate itself, a watt does not need to be followed by a time designation, unless one is talking about a change in power over time, analogous to an acceleration or deceleration.
Because a joule as a quantity of energy does not have a readily-imagined size to the layperson, the non-SI unit watt-hour, or rather its multiple the 'kilowatt-hour', is frequently used as a unit of energy, especially by energy-supply companies (electricity and natural gas suppliers) which often quote charges by the kilowatt-hour. A kilowatt-hour is the amount of energy equivalent to a power of 1 kilowatt running for 1 hour (3.6 MJ).

See also

★ Volt-ampere

★ 2000 Watt society

★ Conversion of units

★ James Watt

★ Declared net capacity (power plants)

★ Orders of magnitude (power)

★ Power factor

★ Root mean square (RMS)

★ Watt balance

★ Watt-hour

References

1. Douglas Malewicki, (1983), International Human Powered Vehicle Association Scientific Symposium, human power-output graph (about half-way down the page, see data point at 0.5 hours and 0.5 hp which equals 375 watts at a conversion of 750 watts per hp), accessed 2007-09-03.
2. Amps, Volts, Watts, Ohms

External links

★ Nelson, Robert A., "''The International System of Units Its History and Use in Science and Industry''". Via Satellite, February 2000.