Watt (unit)

The watt (symbol W) is the SI unit of power, and is defined as the power which in one second gives rise to energy of one joule (1 J = 1 W⋅s). The watt is named for the inventor of the steam engine, James Watt (1736-1819).

By the definition of volt, the watt is also the power dissipated in a current of one ampere across a potential of one volt. From the relation which states that dissipated power P is proportional to electric current i and voltage difference V,

$P=iV,\,$

follows

1 W = 1 ampere·volt ≡ 1 A·V.

The watt is a derived unit in the SI:

${\textrm {W}}={\textrm {J}}\cdot {\textrm {s}}^{-1}=({\textrm {N}}\cdot {\textrm {m}})\cdot {\textrm {s}}^{-1}={\textrm {m}}^{2}\cdot {\textrm {kg}}\cdot {\textrm {s}}^{-3}\equiv {\frac {{\textrm {m}}^{2}\cdot {\textrm {kg}}}{{\textrm {s}}^{3}}},$

where the rightmost side is in terms of SI base units.

Practical use

A healthy adult human can generate about 900 watts sprinting on a cycle. Automobiles generally have a power output from 40 to 200 kilowatts (kW). A Caterpillar D9 bulldozer has a flywheel power of about 305 kW.

Incandescent light bulbs for room illumination typically draw between 40W and 150W of electric power. A solar panel 1 m² in area in direct sunlight receives about 750 watts of solar radiation, and outputs about 120 watts of electric power. Nuclear power stations generate about 0.5 to 2.0 gigawatts (GW) of electrical power; the Three Gorges Dam is projected to supply 22.5 GW when fully operational.