What is a kilowatt-hour?
Energy delivered by electric utilities is usually expressed and charged for in kWh, a unit of energy. The International System of Units unit of energy is the joule (J), equal to one watt – second; one kilowatt hour is exactly 3.6 megajoules, which is the amount of energy transferred if work is done at a rate of one thousand watts for one hour. The kilowatt hour is a convenient unit for electrical bills because the energy usage of a typical electrical customer in one month is several hundred kilowatt hours. Megawatt hours and terawatt hours are used for metering larger amounts of electrical energy. Average annual power production or consumption can be expressed in kilowatt-hours per year; for example, when comparing the energy efficiency of household appliances whose power consumption varies with time or the season of the year, or the energy produced by a distributed power source.
A kilowatt-hour is a unit of energy. Power, or energy converted per second, is measured in watts. A kilowatt is 1000 watts, which is roughly the power consumption rate of devices like electric water heaters, fridges, microwaves and kettles. A kilowatt-hour refers to a 1000 watt device operating continuously for 1 hour. A 500 watt solar pump will produce a kilowatt-hour of electrical energy every 2 hours or around 2.5 kilowatt-hours every day. Energy is often measured in kilowatt-hours because it leads to convenient numbers. For example, we might just as easily measure energy in watt-seconds, but now the solar pump above is producing 9,000,000 watt-seconds of energy per day. Most people find it easier to think in smaller numbers. If you are on scheme power your power bill will quote the average number of ‘units’ you use every day. These units are actually kilowatt-hours.
