Watt

Watts (W) are the yardstick for measuring power. For example, a one hundred watt light bulb is rated to consume one hundred watts of power when turned on - if the light bulb were on for four hours, it would consume a total of 400 watt-hours (Wh) of energy. Watts (W) measure instantaneous power, while watt-hours (Wh) measure the total amount of energy consumed over a period of time.

Kilowatt, Megawatt, and Gigawatt
A kilowatt (kW) is one thousand watts (equal to ten 100-watt light bulbs). A megawatt (MW) is one million watts.

A megawatt hour (MWh) is a measure of the actual amount of power consumed or produced by one megawatt expended for a period of one hour. One MWh is equivalent to the amount of electricity needed to light 10,000 100-watt light bulbs for a one-hour period, or one million watts used for one hour. The formula used to calculate megawatt-hours is: Megawatts (MW) x Hours (h) = Megawatt hours (MWh).

An average U.S. household uses about 10,000 kilowatt-hours (kWh) of electricity each year. If one home needs 1 kWh of energy for one hour, then 1 MWh of energy can sustain 1000 homes for one hour.

A gigawatt is equal to one billion (109) watts or 1 gigawatt = 1000 megawatts. The unit is sometimes 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 and the installed capacity of wind power in Germany was 25.8 GW. The largest unit (out of four) of the Belgian Nuclear Plant Doel has a peak output of 1.04 GW. Though obscure, the "j" sound is still an accepted pronunciation.

Ratio, load power, and peak demand
The ratio of a power plant's average production to its rated capability is known as capacity factor. For example, the wind farm rated at 100 MW capability but averaging 50 MW annual production would have a 50 percent capacity factor (a coal plant with a 750 MW average divided by 1,000 MW rated capability would have a 75 percent capacity factor).

Load factor is calculated by dividing the average energy use by the peak energy use over a certain period of time. If the residential load at a utility averaged 5,000 MW over the course of a year and the peak load was 10,000 MW, then the residential customers would be said to have a load factor of 50 percent (5,000 MW average divided by 10,000 MW peak). Power systems are designed to serve the peak load. Residential homes tend to have low load factors because people are home only during certain hours of the day, while certain industrial customer will have very high load factors because they operate 24 hours a day, 7 days a week.

Peak demand, peak load or on-peak are terms used in energy demand management describing a period in which electrical power is expected to be provided for a sustained period at a significantly higher than average supply level. Peak demand fluctuations may occur on daily, monthly, seasonal and yearly cycles. For an electric utility company, the actual point of peak demand is a single half hour or hourly period which represents the highest point of customer consumption of electricity.

Power sources are often described by the peak MW power they could produce and the average they actually produce. A 100 MW rated wind farm, for example, is capable of producing 100 MW during peak winds, but will probably produce less when winds are light. As a result, over a year a 100MW-rated wind farm may average 50 MW of power production. Similarly, a 1,000 MW coal plant may average 750 MW of production over the course of a year because the plant will shut down for maintenance and/or the plant operates at less than its rated capability, such as when other power plants can produce power less expensively.

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