Watt

For other uses, see Watt (disambiguation)}}}.

The watt (symbol: W) is the SI derived unit of power. A typical household, incandescent lightbulb uses 100 watts. The Hoover Dam produces about two billion watts.

Contents

1 Definition
2 Origin
3 SI multiples
4 Derived and qualified units for power distribution

4.1 Kilowatt-hour
4.2 MWe, MWt
4.3 Peak watt (or Watt peak)

5 See also
6 External links

Definition

One watt is one joule of energy per second.

1 W = 1 J/s = 1 newton meter per second = 1 kg·m²·s⁻³

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

Multiple	Name	Symbol	Multiple	Name	Symbol
10⁰	watt	W
10¹	decawatt	daW	10^–1	deciwatt	dW
10²	hectowatt	hW	10^–2	centiwatt	cW
10³	kilowatt	kW	10^–3	milliwatt	mW
10⁶	megawatt	MW	10^–6	microwatt	µW
10⁹	gigawatt	GW	10^–9	nanowatt	nW
10¹²	terawatt	TW	10^–12	picowatt	pW
10¹⁵	petawatt	PW	10^–15	femtowatt	fW
10¹⁸	exawatt	EW	10^–18	attowatt	aW
10²¹	zettawatt	ZW	10^–21	zeptowatt	zW
10²⁴	yottawatt	YW	10^–24	yoctowatt	yW

Derived and qualified units for power distribution

Kilowatt-hour

A kilowatt-hour (kWh) is a unit of energy, equal to 3,600,000 joules or 3.6 MJ (Joule being the proper unit of energy) or the energy that would be transferred at a constant rate of one kilowatt for one hour. This unit is often used in the context of power plants and home energy bills.

Note that, since 1 W = 1 J/s, the Wh mixes up two measurements of time, of which the hour is not an SI unit. It first divides by seconds and then multiplies by hours. Another unit that is sometimes used is the kWh/yr (kiloWatthour/year), which mixes up three units of time (second, hour and year). For conversion purposes:

1 Wh = 3600 J

:so 1 kWh = 3,6 · 10⁶J = 3600 kJ

1 Wh/yr = 0,000019 W

:so 1 kWh/yr = 0,019 W = 19 mW

MWe, MWt

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). This is a term that refers to thermal power produced. SI prefixes can be used, for example megawatt thermal (MWt) and gigawatt thermal (GWt). For example, a nuclear power plant might use a fission reactor to generate heat (thermal output) which creates steam to drive a turbine to generate electricity.

Peak watt (or Watt peak)

Since solar cell output power depends on multiple factors, such as the sun's incidence angle, for comparison purposes between different cells and panels, the peak watt (Wp) is used. It is the output power under these conditions: [link]

solar irradiance 1000 W/m²
solar reference spectrum AM (airmass) 1.5
cell temperature 25°C

This SI unit is named after James Watt. As for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (W). But when an SI unit is spelled out, it should always be written in lowercase (watt), unless it begins a sentence or is the name "degree Celsius".
— Based on [The International System of Units], section 5.2.

External links

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

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