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 the rate of about 200 watts. 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 energy-saving compact fluorescent lights, which are beginning to replace incandescent bulbs, typically consume 8 to 20 watts.

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Definition

In electrical terms, it follows that:

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 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: W_e) is a term that refers to power produced as electricity, while watt thermal (abbreviation: W_t or W_th) 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 (MW_e), gigawatt electrical (GW_e), megawatt thermal (MW_t) and gigawatt thermal (GW_t). 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", which makes little 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, 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

References