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# Watt

### From Encyclopedia PRO

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 |

## Definition

One watt is one joule of energy per second.

## 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

## 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 · 10
^{6}J = 3600 kJ

- so 1 kWh = 3,6 · 10

- 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: [1]

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