Watt

==Distinction between watts and watt-hours==
The terms [[Power (physics)|power]] and [[energy]] are closely related but distinct physical quantities. 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 [[Electric light|light bulb]] with a [[power rating]] of {{gaps|100|W}} is turned on for one hour, the energy used is 100 [[watt hour]]s (W&middot;h), 0.1 kilowatt hour, or 360&nbsp;[[joule#Kilojoule|kJ]]. This same amount of energy would light a 40-watt bulb for 2.5 hours, or a 50-watt bulb for 2 hours.

[[Power station]]s are rated using units of power, typically megawatts or gigawatts (for example, the [[Three Gorges Dam]] in China, 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 hour]]s 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:

: Power output = energy / time

: 1 terawatt hour per year = 1&times;10<sup>12</sup> W·h / (365 days &times; 24 hours per day) ≈ 114 million watts,
equivalent to approximately 114 megawatts of constant power output.

The [[watt-second]] is a unit of energy, equal to the [[joule]]. One kilowatt hour is 3,600,000 watt seconds.

While a watt per hour is a unit of rate of change of power with time,{{efn-lr| Watts per hour refers to the ''rate of change'' of power being used (or generated). For example, a power plant that changes its power output from 100&nbsp;MW to 200&nbsp;MW in 15 minutes would have a ramp-up rate of 400&nbsp;MW/h. Gigawatts per hour are used to characterize the ramp-up required of the [[power plant]]s on an electric grid to compensate for loss of output from other sources, such as when [[solar power]] generation drops to zero as the sun sets. See [[duck curve]].}} it is not correct to refer to a watt (or watt-hour) as a watt per hour.<ref>{{cite web |url=http://www.windsun.com/Inverters/Inverter_selection.htm |title=Inverter Selection |publisher=Northern Arizona Wind and Sun |access-date=27 March 2009 |url-status=live |archive-url=https://web.archive.org/web/20090501140617/http://www.windsun.com/Inverters/Inverter_selection.htm |archive-date=1 May 2009 }}</ref>