Watt

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 a rate of about 200 watts. An 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 energy-saving compact fluorescent lights, which are gradually replacing incandescent bulbs, typically consume 5 to 30 watts.

Definition

Failed to parse (Missing texvc executable; please see math/README to configure.): 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}}.

In electrical terms, it follows that:

Failed to parse (Missing texvc executable; please see math/README to configure.): 1~\rm{W} = 1~\rm{A} \times 1~\Omega \times 1~\rm{A} = 1~\rm{V} \cdot \rm{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.^[1]

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. A one bar electric fire might use 1 kilowatt.

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 U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW.^[2]

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).

Electrical and thermal

Megawatt electrical (abbreviation: MW_e^{[citation needed]} or MWe^[3]) is a term that refers to electric power, while megawatt thermal (abbreviation: MW_t or MWt) refers to thermal power produced. Though 'megawatt electrical' and 'megawatt thermal' are not SI units,^[4] alternative SI prefixes are sometimes used, for example gigawatt electrical (GW_e). 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.^[5]

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 MW_t of heat, which creates steam to drive a turbine, which generates 648 MW_e 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",^[6] it makes little sense since a watt is already a rate, specifically representing 1 joule of energy per second. As such, 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, often in its multiples such the kilowatt-hour or higher prefixes, 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:

(1 kW·h)(1000 W/kW)(3600 s/h) = 3,600,000 W·s = 3,600,000 J = 3.6 MJ.

See also

energy Portal

References

External links

• Nelson, Robert A., "The International System of Units Its History and Use in Science and Industry". Via Satellite, February 2000.ar:واط

ast:Vatiu bn:ওয়াট (একক) zh-min-nan:Oá bs:Vat br:Watt bg:Ват ca:Watt cs:Watt da:Watt de:Watt (Einheit) et:Vatt (ühik) el:Βατ (μονάδα μέτρησης) es:Vatio eo:Vato (unuo) eu:Watt fr:Watt gl:Watt ko:와트 hr:Vat id:Watt is:Vatt it:Watt he:ואט ku:Wat lv:Vats lt:Vatas hu:Watt (mértékegység) mk:Ват ms:Watt nl:Watt ja:ワット no:Watt (effekt) nn:Watt pl:Wat pt:Watt ro:Watt ru:Ватт sco:Watt simple:Watt sk:Watt (jednotka) sl:Vat sr:Ват sh:Watt fi:Watti sv:Watt th:วัตต์ vi:Watt tr:Watt uk:Ват ur:واٹ zh-yue:瓦 (單位)