Watt

For other uses, see Watt (disambiguation).

Kilowatts redirects here. For the electronic musician James Watts, see KiloWatts.

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.

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.

$1\ \mathrm {W} =1\ {\dfrac {\mathrm {J} }{\mathrm {s} }}=1\ {\dfrac {\mathrm {kg} \cdot \mathrm {m^{2}} }{\mathrm {s^{3}} }}=1{\dfrac {\mathrm {N\cdot m} }{\mathrm {s} }}.$

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.

Origin

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.

SI multiples

SI multiples of watt (W)

Submultiples			Multiples
Value	SI symbol	Name	Value	SI symbol	Name
10−1 W	dW	deciwatt	101 W	daW	decawatt
10−2 W	cW	centiwatt	102 W	hW	hectowatt
10−3 W	mW	milliwatt	103 W	kW	kilowatt
10−6 W	μW	microwatt	106 W	MW	megawatt
10−9 W	nW	nanowatt	109 W	GW	gigawatt
10−12 W	pW	picowatt	1012 W	TW	terawatt
10−15 W	fW	femtowatt	1015 W	PW	petawatt
10−18 W	aW	attowatt	1018 W	EW	exawatt
10−21 W	zW	zeptowatt	1021 W	ZW	zettawatt
10−24 W	yW	yoctowatt	1024 W	YW	yottawatt
10−27 W	rW	rontowatt	1027 W	RW	ronnawatt
10−30 W	qW	quectowatt	1030 W	QW	quettawatt

Derived and qualified units for power distribution

Kilowatt

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

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.

The watt is named after James Watt. As with every SI unit named for a person, its symbol starts with an upper case letter (W), but when written in full, it follows the rules for capitalisation of a common noun; i.e., watt becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.

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

External links

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