The watt (symbol: W) is a unit of power. In the International System of Units (SI) it is defined as a derived unit of 1 joule per second,^{[1]} and is used to quantify the rate of energy transfer. In dimensional analysis it is described by .
When an object's velocity is held constant at one meter per second against a constant opposing force of one newton, the rate at which work is done is 1 watt.
In terms of electromagnetism, one watt is the rate at which electrical work is performed when a current of one ampere (A) flows across an electrical potential difference of one volt (V).
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.
The watt is named after the Scottish inventor James Watt for his contributions to the development of the steam engine. The measurement unit was recognized by the Second Congress of the British Association for the Advancement of Science in 1882, concurrent with the start of commercial power production from both water and steam. In 1960 the 11th General Conference on Weights and Measures adopted it for the measurement of power into the International System of Units (SI).

The attowatt (aW) is equal to 10^{−18} watt. The sound intensity in water corresponding to the international standard reference sound pressure of 1 μPa is approximately 0.65 aW/m^{2}.^{[3]}
The femtowatt (fW) is equal to one quadrillionth (10^{−15}) of a watt. Technologically important powers that are measured in femtowatts are typically found in reference(s) to radio and radar receivers. For example, meaningful FM tuner performance figures for sensitivity, quieting and signaltonoise require that the RF energy applied to the antenna input be specified. These input levels are often stated in dBf (decibels referenced to 1 femtowatt). This is 0.2739 microvolt across a 75ohm load or 0.5477 microvolt across a 300ohm load; the specification takes into account the RF input impedance of the tuner.
The picowatt (pW), not to be confused with the much larger petawatt (PW), is equal to one trillionth (10^{−12}) of a watt. Technologically important powers that are measured in picowatts are typically used in reference to radio and radar receivers, acoustics and in the science of radio astronomy. One picowatt is the international standard reference value of sound power when this quantity is expressed as a level in decibels.^{[4]}
The nanowatt (nW) is equal to one billionth (10^{−9}) of a watt. Important powers that are measured in nanowatts are also typically used in reference to radio and radar receivers.
The microwatt (µW) is equal to one millionth (10^{−6}) of a watt. Important powers that are measured in microwatts are typically stated in medical instrumentation systems such as the EEG and the ECG, in a wide variety of scientific and engineering instruments and also in reference to radio and radar receivers. Compact solar cells for devices such as calculators and watches are typically measured in microwatts.^{[5]}
The milliwatt (mW) is equal to one thousandth (10^{−3}) of a watt. A typical laser pointer outputs about five milliwatts of light power, whereas a typical hearing aid for people uses less than one milliwatt.^{[6]} Audio signals and other electronic signal levels are often measured in dBm, referenced to one milliwatt.
The kilowatt (kW) is equal to one thousand (10^{3}) watts. This unit is typically used to express the output power of engines and the power of electric motors, tools, machines, and heaters. It is also a common unit used to express the electromagnetic power output of broadcast radio and television transmitters.
One kilowatt is approximately equal to 1.34 horsepower. A small electric heater with one heating element can use 1.0 kilowatt. The average electric power consumption of a household in the United States is about one kilowatt.^{[ii]}
A surface area of one square meter on Earth receives typically about one kilowatt of sunlight from the sun (the solar irradiance) (on a clear day at mid day, close to the equator).^{[8]}
The megawatt (MW) is equal to one million (10^{6}) watts. Many events or machines produce or sustain the conversion of energy on this scale, including large electric motors; large warships such as aircraft carriers, cruisers, and submarines; large server farms or data centers; and some scientific research equipment, such as supercolliders, and the output pulses of very large lasers. A large residential or commercial building may use several megawatts in electric power and heat. 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, uses more than 12 MW, while heavy dieselelectric locomotives typically produce/use 3 to 5 MW. U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW.^{[9]}
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 (GW) is equal to one billion (10^{9}) watts or 1 gigawatt = 1000 megawatts. This unit is often used for large power plants or power grids. For example, by the end of 2010 power shortages in China's Shanxi province were expected to increase to 5–6 GW^{[10]} and the installed capacity of wind power in Germany was 25.8 GW.^{[11]} The largest unit (out of four) of the Belgian Doel Nuclear Power Station has a peak output of 1.04 GW.^{[12]} HVDC converters have been built with power ratings of up to 2 GW.^{[13]}
The terawatt (TW) is equal to one trillion (10^{12}) watts. The total power used by humans worldwide is commonly measured in terawatts (see primary energy). The most powerful lasers from the mid1960s to the mid1990s produced power in terawatts, but only for nanosecond time frames. The average lightning strike peaks at 1 terawatt, but these strikes only last for 30 microseconds.
The petawatt (PW) is equal to one quadrillion (10^{15}) watts and can be produced by the current generation of lasers for time scales on the order of picoseconds (10^{−12} s). One such laser is the Lawrence Livermore's Nova laser, which achieved a power output of 1.25 PW (×10^{15} W) by a process called 1.25chirped pulse amplification. The duration of the pulse was roughly 0.5 ps (×10^{−13} s), giving a total energy of 600 J.^{[14]} Another example is the Laser for Fast Ignition Experiments (LFEX) at the Institute of Laser Engineering (ILE), 5Osaka University, which achieved a power output of 2 PW for a duration of approximately 1 ps.^{[15]}^{[16]}
Based on the average total solar irradiance^{[17]} of 1.366 kW/m^{2}, the total power of sunlight striking Earth's atmosphere is estimated at 174 PW (see: solar constant).
In the electric power industry, megawatt electrical (MWe^{[18]} or MW_{e}^{[19]}) refers by convention to the electric power produced by a generator, while megawatt thermal or thermal megawatt^{[20]} (MWt, MW_{t}, or MWth, MW_{th}) refers to thermal power produced by the plant. For example, the Embalse nuclear power plant in Argentina uses a fission reactor to generate 2109 MW_{t} (i.e. heat), which creates steam to drive a turbine, which generates 648 MW_{e} (i.e. electricity). Other SI prefixes are sometimes used, for example gigawatt electrical (GW_{e}). The International Bureau of Weights and Measures, which maintains the SIstandard, states that further information about a quantity should not be attached to the unit symbol but instead to the quantity symbol (i.e., P_{thermal} = 270 W rather than P = 270 W_{th}) and so these units are nonSI.^{[21]} In compliance with SI the energy company DONG Energy uses the unit megawatt for produced electrical power and the equivalent unit megajoule/s for delivered heating power in a combined heat and power station such as Avedøre Power Station.^{[22]}
When describing alternating current (AC) electricity, another distinction is made between the watt and the voltampere. While these units are equivalent for simple resistive circuits, they differ when loads exhibit electrical reactance.
Radio stations usually report the power of their transmitters in units of watts, referring to the effective radiated power. It refers to the relative power of the transmission when it is directed towards the horizon for maximum geographic coverage, rather than uniformly broadcast in all directions.
The terms power and energy are frequently confused. Power is the rate at which energy is generated or consumed and hence is measured in units (e.g. watts) that represent energy per unit time.
For example, when a light bulb with a power rating of 100W 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 40watt bulb for 2.5 hours, or a 50watt bulb for 2 hours.
Power stations are rated using units of power, typically megawatts or gigawatts (for example, the Three Gorges Dam is rated at approximately 22 gigawatts). This reflects the maximum power output it can achieve at any point in time. A power station's annual energy output, however, would be recorded using units of energy (not power), typically gigawatt hours. Major energy production or consumption is often expressed as terawatt hours for a given period; often a calendar year or financial year. One terawatt hour of energy is equal to a sustained power delivery of one terawatt for one hour, or 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, although the term joule is generally employed.^{[citation needed]}
While a watt per hour exists in principle (as a unit of rate of change of power with time^{[iii]}), it is not correct to refer to a watt (or watt hour) as a "watt per hour".^{[23]}
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