The watt (symbol: W) is a derived unit of power in the International System of Units (SI). It measures rate of energy conversion. One watt is equivalent to 1 joule (J) of energy per second.

In terms of mechanical energy, one watt is the rate at which work is done when an object is moved at a speed of one meter per second against a force of one newton.

By the definitions of electric potential (volt) and current (ampere), work is done at a rate of one watt when one ampere flows through a potential difference of one volt.

Examples of usage

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

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 (SI).

Multiples

Nanowatt

The nanowatt is equal to one billionth of a watt. From a single star of magnitude +3.5 a square meter receives one nanowatt.

Microwatt

The microwatt is equal to one millionth of a watt.

Milliwatt

The milliwatt is equal to one thousandth of a watt. A typical laser pointer might output 5 milliwatts.

Kilowatt

The kilowatt 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 annual electrical energy consumption of a household in the United States is about 8,900 kilowatt-hours, equivalent to an average power of about 1 kW.

Megawatt

The megawatt is equal to one million 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. On railways, modern high-powered electric locomotives typically have a peak power output of 5 or 6 MW although some produce much more - the Eurostar, for example, produces more than 12 MW - while heavy diesel-electric locomotives typically manage 3 to 5 MW, whereas U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW.

The earliest citing for "megawatt" in the Oxford English Dictionary is a reference in the 1900 Webster's International Dictionary of English Language. The OED also says "megawatt" appeared in a 28 November 1947 article in Science (506:2).

Gigawatt

The gigawatt is equal to one billion watts. This unit is sometimes used with large power plants or power grids.

Terawatt

The terawatt is equal to one trillion watts. The total power used by humans worldwide (about 16 TW in 2006) is commonly measured in these units. The most powerful lasers from the mid 1960s to the mid 1990s produced power in terawatts, but only for nanoseconds. The average stroke of lightning peaks at 1 terawatt, but these strokes only last for 30 microseconds.

Petawatt

The petawatt is equal to one quadrillion watts and can be produced by the current generation of lasers for time-scales of the order of femtoseconds (10^-15 s). Based on the average of 1.366 kW/m² of total solar irradiance the total energy flow of sunlight striking Earth's atmosphere is estimated at 174 PW (cf. Solar Constant). If all this power were absorbed this would be equivalent to the Earth gaining mass at a rate of 1.94 kg/s.

Electrical and thermal watts

In the electric power industry, megawatt electrical (abbreviation: MW_e or MWe) is a term that refers to electric power, while megawatt thermal or thermal megawatt (abbreviations: MW_t, MW_th, MWt, or MWth) refers to thermal power produced. Other SI prefixes are sometimes used, for example gigawatt electrical (GW_e).

For example, the Embalse nuclear power plant in Argentina uses a fission reactor to generate 2109 MW_t of heat, which creates steam to drive a turbine, which generates 648 MW_e of electricity. The difference is due to the inefficiency of steam-turbine generators and the limitations of the theoretical Carnot Cycle.

Confusion of watts, watt-hours, and watts per hour

Power and energy are frequently confused. Power is the rate at which energy is generated and consumed. For example, if a light bulb is turned on for one hour, the energy used is 100 watt-hours (W·h) or 0.1 kilowatt-hour, or 360 kJ. This same quantity of energy would light a 40-watt bulb for 2.5 hours, or a 50-watt bulb for 2 hours. A power station would be rated in watts, but its annual energy sales would be in watt-hours (or kilowatt-hours or megawatt-hours). A kilowatt-hour is the amount of energy equivalent to a steady power of 1 kilowatt running for 1 hour, or 3.6 MJ.

Terms such as 'watts per hour' are often misused. Watts per hour properly refers to the change of power per hour. Watts per hour (W/h) is useful to characterize the ramp-up speed of power plants. For example, a power plant that reaches a power of 1 MW from zero in 15 minutes has a ramp-up rate of 4 MW/h. Hydroelectric power plants have a very high ramp-up speed, which makes them particularly useful in peak load and emergency situations.

Major energy production or consumption for a period is often expressed as terawatt-hours produced or consumed during the period. The period used is often a calendar year or a financial year. A terawatt-hour equates to a continuous energy production or consumption of approximately 114 megawatts for a period of one year.

See also