How is Electricity Measured?

Understanding watts, megawatts, kilowatt-hours, and more

Watts are a measurement of power, describing the rate at which electricity is being used at a specific moment. For example, a 15-watt LED light bulb draws 15 watts of electricity at any moment when turned on.

Watt-hours are a measurement of energy, describing the total amount of electricity used over time. Watt-hours are a combination of how fast the electricity is used (watts) and the length of time it is used (hours). For example, a 15-watt light bulb, which draws 15 watts at any one moment, uses 15 watt-hours of electricity in the course of one hour.

Kilowatts and kilowatt-hours are useful for measuring amounts of electricity used by large appliances and by households. Kilowatt-hours are what show up on your electricity bill, describing how much electricity you have used. One kilowatt (kW) equals 1,000 watts, and one kilowatt-hour (kWh) is one hour of using electricity at a rate of 1,000 watts. New, energy-efficient refrigerators use about 300-400 kilowatt-hours per year. The typical American home uses about 7,200 kilowatt-hours of electricity each year [1].

Megawatts are used to measure the output of a power plant or the amount of electricity required by an entire city. One megawatt (MW) = 1,000 kilowatts = 1,000,000 watts. For example, a typical coal plant is about 600 MW in size.

Gigawatts measure the capacity of large power plants or of many plants. One gigawatt (GW) = 1,000 megawatts = 1 billion watts. In 2012, the total capacity of U.S. electricity generating plants was approximately 1,100 GW [2].

Notes and References

[1] Assumes a typical U.S. household uses non-electric heating.

[2] SNL Financial. Historic & Future Power Plant Capacity.

We Need Your Support
to Make Change Happen

We can shift our nation away from dirty fossil fuels and toward cleaner, renewable sources of power—but not without you. Your generous support helps develop science-based solutions for a healthy, safe, and sustainable future.