Watt
The watt (symbol: W) is the SI derived unit of power, equal to one joule of energy per second. It measures a rate of energy use or production.
A human climbing a flight of stairs is doing work at a rate of about 200 watts.  A typical automobile engine produces mechanical energy at a rate of 25,000 watts (approximately 33.5 horsepower) while cruising.  A typical household incandescent light bulb uses electrical energy at a rate of 25 to 100 watts, while compact fluorescent lights typically consume 5 to 30 watts.

# Definition
1~\rm{W} = 1~\dfrac{\rm{J}}{\rm{s}} = 1~\dfrac{\rm{kg} \cdot \rm{m^2}}{\rm{s^3}} = 1~\dfrac{\rm{N\cdot m}}{\rm{s}} \,
In electrical terms, it follows that:
Or, in terms of volts and amperes:
1~\rm{W} = 1~\rm{V} \times 1~\rm{A} \,
That is, if 1 volt of potential difference is applied to a resistive load, and a current of 1 ampere flows, then 1 watt of power is dissipated.  More simply stated: watts is equal to amps times volts.
Note that the electrical definitions are true instantaneously, and for DC voltage and current. The Volt-ampere article explains the consequences when the  RMS voltage and current are measured separately.

# 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 (or "SI").

# Derived and qualified units for power distribution

## 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 roughly equivalent to 1.34 horsepower. An electric heater with one heating-element might use 1 kilowatt.

## 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 the supercollider and large lasers). A large residential or retail 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.
According to the Oxford English Dictionary, the earliest citing for "megawatt" is a reference in the 1900 Webster's International Dictionary of English Language. The OED also says "megawatt" appeared in a 28 November, 1847, article in Science (506:2).

## Gigawatt
The gigawatt (symbol: GW) is equal to one billion (109) watts. This unit is sometimes used with large power plants or power grids.

## Terawatt
The terawatt (symbol: TW) is equal to one trillion (1012) watts. The average energy usage of the earth (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.

## Electrical and thermal
Megawatt electrical (abbreviation: MWe or MWe) is a term that refers to electric power, while megawatt thermal (abbreviations: MWt, MWth, MWt, or MWth) refers to thermal power produced.  Though 'megawatt electrical' and 'megawatt thermal' are not SI units, alternative SI prefixes are sometimes used, for example gigawatt electrical (GWe). The International Bureau of Weights and Measures states that unit symbols should not use subscripts to provide additional information about the quantity being measured, and regards these symbols as incorrect.
These terms are used by engineers to disambiguate the electric output of a thermal power station versus the (larger) thermal output.  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 heat lost to the surroundings.

# Confusion of watts and watt-hours
Power and energy are frequently confused in the general media.  A watt is one 1 joule of energy per second.  So watts multiplied by a period of time equals energy.  For example, if a 100 watt light bulb is turned on for one hour, then an amount of energy is used corresponding to 100 watts of power being generated for a time period of one hour, i.e. 100 watts times one hour, i.e. 0.1 kilowatt-hour.
Since a joule as a quantity of energy does not have a readily imagined size to the layperson, the non-SI unit watt-hour, often in its multiples such the kilowatt-hour or higher prefixes, is frequently used as a unit of energy, especially by energy-supply companies (electricity and natural gas suppliers), which often quote charges by the kilowatt-hour. A kilowatt-hour is the amount of energy equivalent to a power of 1 kilowatt running for 1 hour: