What Is a kWh?

A kWh, or kilowatt hour, is a unit of energy equal to one kilowatt of power sustained for one hour. It is the standard commercial unit for metering and billing electrical energy, and it is the quantity that appears on a utility invoice, on the nameplate of a battery pack, and in national energy statistics. Because power is the rate at which energy is transferred, multiplying a power in kilowatts by a duration in hours yields an energy in kilowatt hours: a 2 kW space heater running for three hours consumes 6 kWh.

The unit is not itself part of the International System of Units, but it combines legitimately with SI quantities because the watt is an SI derived unit and the hour is a non-SI unit accepted for use with the SI. One kilowatt hour equals exactly 3.6 megajoules, since an hour contains 3,600 seconds and a joule is one watt-second. In the older units still common in North American thermal accounting, one kWh is about 3,412 British thermal units, a conversion the US Energy Information Administration publishes alongside its other energy unit conversions so that electricity, natural gas, and fuel oil can be compared on a common basis.

Power Versus Energy

The most persistent confusion around the unit is the distinction between the kilowatt and the kilowatt hour. A kilowatt is an instantaneous rate; a kilowatt hour is an accumulated quantity. Generator nameplates, transformer ratings, and demand charges are expressed in kilowatts or megawatts because they describe how much power the equipment can handle at any moment. Fuel purchases, generation totals, and residential bills are expressed in kilowatt hours because they describe how much energy actually moved. The EIA's explanation of how electricity is measured sets out this separation directly, and it matters commercially: many industrial tariffs charge separately for peak demand in kilowatts and for consumption in kilowatt hours.

Metering and Measurement

Electricity meters integrate instantaneous power over time to produce a kilowatt hour reading. Traditional induction meters did this mechanically, using an aluminum disc whose rotation rate is proportional to power and whose revolution count is proportional to energy. Solid-state meters sample voltage and current waveforms at high rates, compute real power from their product, and accumulate the result digitally, which also allows them to record reactive energy in kilovolt-ampere reactive hours and to timestamp consumption for time-of-use tariffs. Revenue-grade meters are held to accuracy classes typically within 0.2 to 0.5 percent, and utilities recalibrate or sample-test them under regulatory schedules because billing accuracy across millions of customers depends on it.

Multiples and System-Scale Accounting

The kilowatt hour scales through the usual metric prefixes. A megawatt hour is 1,000 kWh, a gigawatt hour is one million, and a terawatt hour is one billion. National and regional statistics are normally reported in gigawatt hours or terawatt hours, and the EIA's accounting of United States generation, capacity, and sales uses these multiples to separate installed capacity, measured in megawatts, from annual output, measured in megawatt hours. Derived metrics inherit the unit as a denominator: levelized cost of energy is quoted in dollars per megawatt hour, grid carbon intensity in grams of carbon dioxide per kilowatt hour, and electric vehicle efficiency in kilowatt hours per 100 miles.

Applications

The kilowatt hour has applications in a range of fields, including:

• Utility metering, tariff design, and retail electricity billing
• Battery and energy storage capacity specification
• Building energy codes and efficiency benchmarking
• Electric vehicle range and charging cost calculation
• Renewable project finance, where output is contracted per megawatt hour
• Greenhouse gas inventories that report emissions per unit of electricity