The watt is a derived unit of power in the International System of Units (SI), named after the 18thcentury Scottish engineer James Watt. Its unit symbol is W. The unit measures the rate of energy conversion.
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A person having a mass of 100 kilograms climbs a 3 meter high ladder in 5 seconds is doing work at a rate of about 600 watts. Mass x acceleration due to gravity x height divided by the time it takes to lift the mass to the given height gives the rate of doing work or the power.
A typical passenger automobile engine yields a power output of 25000watts while cruising. 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, while comparable LED lamps use about 0.5 to 6 watts. A typical coal powered power station produces around 600700 megawatts.
The watt is named after James Watt for his contributions to the development of the steam engine. The unit was recognized by the Second Congress of the British Association for the Advancement of Science in 1889. The 11th General Conference on Weights and Measures in 1960 adopted it for the measurement of power into the International System of Units (SI).

The picowatt is equal to one trillionth of a watt. Technologicallyimportant powers that are measured in picowatts are typically found in radio and radar receivers, and also in the science of radio astronomy.
The nanowatt is equal to one billionth of a watt. A surface area of one square meter on Earth receives one nanowatt of power from a single star of apparent magnitude +3.5. Important powers that are measured in nanowatts are also typically found in radio]] and radar receivers
The microwatt is equal to one millionth of a watt. Important powers that are measured in microwatts are typically found in medical instrumentation systems such as the EEG and the EKG, in a wide vatriety of scientific and engineering instruments and also in radio and radar receivers
The milliwatt is equal to one thousandth of a watt. A typical laser pointer outputs about five milliwatts of light power, whereas a typical hearing aid for people consumes less than one milliwatt.^{[1]}
The kilowatt is equal to one thousand watts. This unit is typically used to express the output power of engines and the power consumption of tools and machines. It is also a common unit used to express the electromagnetic power output of radio transmitters.
One kilowatt of power is very close 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 kilowatthours, equivalent to a steady power consumption of about 1 kW, for an entire year,^{[2]} Also, kilowatts of light power can be measured in the output pulses of some lasers.
The megawatt is equal to one million watts. Many events or machines produce or sustain the conversion of energy on this scale. For example: lightning strikes, large electric motors, large warships, such as aircraft carriers, cruisers, and submarines, engineering hardware, and some scientific research equipment, such as supercolliders, and in the output pulses of very large lasers. A large residential or commercial building may consume several megawatts in electric power and heat.
The productive capacity of electrical generators operated by a utility company is often measured in MW. On railways, modern highpowered 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 dieselelectric locomotives typically consume 3 to 5 MW. U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW.^{[3]}
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).
The gigawatt is equal to one billion watts. This unit is sometimes used for large power plants or power grids. For example, in 2009, the installed capacity of wind power in Germany was 25 GW. The Nuclear Plant Doel has a peak output of 3.0 GW.
The terawatt is equal to one trillion watts. The total power used by humans worldwide (about 16 TW in 2006) is commonly measured in this unit. The most powerful lasers from the mid1960s to the mid1990s produced power in terawatts, but only for nanosecond time frames. The average stroke of lightning peaks at 1 terawatt, but these strokes only last for 30 microseconds.
The petawatt is equal to one quadrillion watts and can be produced by the current generation of lasers for timescales of the order of femtoseconds (10^{−15} s). Based on the average of 1.366 kW/m^{2} of total solar irradiance^{[4]} the total energy flow of sunlight striking Earth's atmosphere is estimated at 174 PW (cf. Solar Constant). If this power was completely absorbed, it would be equivalent to the Earth gaining mass at a rate of 1.94 kg/s.
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}).^{[notes 1]}
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 steamturbine generators and the limitations of the theoretical Carnot Cycle.
Power and energy are frequently confused. Power is the rate at which energy is generated and consumed.
For example, when a light bulb with a power rating of 100W is turned on for one hour, the energy used is 100 watthours (W·h), 0.1 kilowatthour, or 360 kJ. This same amount of energy would light a 40watt bulb for 2.5 hours, or a 50watt bulb for 2 hours. A power station would be rated in multiples of watts, but its annual energy sales would be in multiples of watthours. A kilowatthour 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.^{[8]} Watts per hour properly refers to the change of power per hour. Watts per hour (W/h) might be useful to characterize the rampup 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 rampup rate of 4 MW/h. Hydroelectric power plants have a very high rampup rate, which makes them particularly useful in peak load and emergency situations.
Major energy production or consumption is often expressed as terawatthours for a given period that is often a calendar year or financial year. One terawatthour is equal to a sustained power of approximately 114 megawatts for a period of one year.

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