watt

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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.

1W = 1Js^-1 = 1kgm²s^-3 = 1Nms^-1

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.^[1]

1W=1V×1A

Examples of usage

A human climbing a flight of stairs is doing work at a rate of about 200watts. A typical automobile engine produces mechanical energy at a rate of 25000watts (approximately 33.5 horsepower) while cruising. A typical household incandescent light bulb uses electrical energy at a rate of 25 to 100 watts; compact fluorescent lights typically consume 5 to 30 watts, and LED lamps typically use between 1 and 5 watts.

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

For additional examples of magnitude for multiples and submultiples of the Watt, see Orders of magnitude (power)

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 annual power consumption of a household in the United States is about 8,900 kilowatt-hours.^[2]

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.^[3]

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 (symbol: GW) is equal to one billion (short scale) (10⁹) watts. This unit is sometimes used with large power plants or power grids.

Terawatt

The terawatt (symbol: TW) is equal to one trillion (short scale) (10¹²) watts. The total power used by humans worldwide (about 15 TW) 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 (symbol: PW) is equal to one quadrillion (short scale) (10¹⁵) watts and can be produced by the current generation of lasers for time-scales of the order of femtoseconds (10^-15). Based on the average of 1.366kW/m² of total solar irradiance^[4] the total energy flow of sunlight striking Earth's atmosphere is estimated at 174PW (cf. Sunlight#Solar constant).

Electrical and thermal watts

In the electric power industry, megawatt electrical (abbreviation: MW_e^[5] or MWe^[6]) is a term that refers to electric power, while megawatt thermal or thermal megawatt^[7](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).^[8]

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 and watt-hours

Power and energy are frequently confused by the general public. Power is the rate at which energy is generated and consumed. For example, if a 100W 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. 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 meaningless in practice,^[9] unless referring to 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 situations.

Often major energy production or consumption is expressed as terawatt-hours per year. This equates to the continuous energy production of approximately 114 megawatts for the period of one year.

See also

Energy portal

References

External links

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