The watt ( ; symbol: W) is a derived unit of power in the International System of Units (SI), named after the Scottish engineer James Watt (1736–1819). The unit, defined as one joule per second, measures the rate of energy conversion.
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:Two additional unit conversions for watt can be found using the above equation and Ohm's Law. : :Where ohm () is the SI derived unit of electrical resistance.
A medium-sized passenger automobile engine is rated at 50–150 kilowatts – while cruising it will typically yield half that amount. A typical household incandescent light bulb has a power rating of 25 to 100 watts; fluorescent lamps typically consume 5 to 30 watts to produce a similar amount of light.
A typical coal power station produces around 600-700 megawatts.
One kilowatt of power is approximately equal to 1.34 horsepower. A small electric heater with one heating element can use 1.0 kilowatt. The average annual electrical energy consumption of a household in the United States is about 8,900 kilowatt-hours (cf the average UK household's approx 4,700 kilowatt-hours for example), equivalent to a steady power consumption of about 1 kW for an entire year. Also, kilowatts of light power can be measured in the output pulses of some lasers.
The productive capacity of electrical generators operated by a utility company 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, consumes more than 12 MW—while heavy diesel-electric locomotives typically produce/consume 3 to 5 MW. U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW.
The earliest citing of the megawatt in the Oxford English Dictionary (OED) is a reference in the 1900 Webster's International Dictionary of English Language. The OED also states that megawatt appeared in a 28 November 1947 article in the journal Science (506:2).
In the 1985 movie, Back to the Future, Dr. Emmitt Brown needs a nuclear reaction to generate the 1.21 gigawatts needed to send the Delorean time machine back to the future. He harnesses it through a bolt of lightning.
Though “gigawatt” is usually pronounced today with a hard initial "g", the “j” variant is also accepted (see giga-#Pronunciation).
==Electrical and thermal watts== In the electric power industry, megawatt electrical (abbreviation: MWe or MWe) is a term that refers to electric power, while megawatt thermal or thermal megawatt (abbreviations: MWt, MWth, MWt, or MWth) refers to thermal power produced. Other SI prefixes are sometimes used, for example gigawatt electrical (GWe).
For example, the Embalse nuclear power plant in Argentina uses a fission reactor to generate 2109 MWt of heat, which creates steam to drive a turbine, which generates 648 MWe of electricity. The difference is due to the inefficiency of steam-turbine generators and the limitations of the theoretical Carnot Cycle.
For example, when a light bulb with a power rating of is turned on for one hour, the energy used is 100 watt-hours (W•h), 0.1 kilowatt-hour, or 360 kJ. This same amount 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 multiples of watts, but its annual energy sales would be in multiples of watt-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) might be useful to characterize the ramp-up behavior of power plants. For example, a power plant that reaches a power output of 1 MW from 0 MW in 15 minutes has a ramp-up rate of 4 MW/h. Hydroelectric power plants have a very high ramp-up rate, which makes them particularly useful in peak load and emergency situations.
Major energy production or consumption is often expressed as terawatt-hours for a given period that is often a calendar year or financial year. One terawatt-hour is equal to a sustained power of approximately 114 megawatts for a period of one year.
The watt second is a unit of energy, equal to the joule. One kilowatt-hour is 3,600,000 watt-seconds. The watt-second is used, for example, to rate the energy storage of flash lamps used in photography.
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