The watt (symbol: W) is the SI derived unit of power, equal to one joule of energy per second. It measures a rate of energy conversion.

A human climbing a flight of stairs is doing work at a rate of about 200 watts. A typical automobile engine produces mechanical energy at a rate of 25,000 watts (approximately 33.5 horsepower) while cruising. A typical household incandescent light bulb uses electrical energy at a rate of 25 to 100 watts, while compact fluorescent lights typically consume 5 to 30 watts.

One watt is the rate at which work is done when an object is moving at one meter per second against a force of one newton.

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

By the definition of the units of ampere and volt, work is done at a rate of one watt when one ampere flows through a potential difference of one volt.

1~\rm{W} = 1~\rm{V \cdot } 1~\rm{ A}

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").

Nanowatt

The nanowatt is equal to one billionth (10^-9) of a watt. From a single star of magnitude +3.5 a square meter receives one nanowatt.

Microwatt

The microwatt (symbol:μW) is equal to one millionth (10^-6) of a watt.

Milliwatt

The milliwatt (symbol:mW) is equal to one thousandth (10^-3) of a watt. A typical laser pointer might output 5 milliwatts.

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 approximately equivalent to 1.34 horsepower. An electric heater with one heating element might use 1 kilowatt. The average power consumption, over a year, of a household in the United States is about 8,900 kilowatt-hours.

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 supercolliders and large lasers). A large residential or commercial 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 U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW.