Electricity is measured in Watts and kilowatts

A mechanical electricity meter
Picture of a residential electricity meter.

Source: Stock photography (copyrighted)

A smart electricity meter
Picture of a smart meter.

Source: Stock photography (copyrighted)

Electricity is measured in units of power called Watts, named to honor James Watt, the inventor of the steam engine. A Watt is the unit of electrical power equal to one ampere under the pressure of one volt.

One Watt is a small amount of power. Some devices require only a few Watts to operate, and other devices require larger amounts. The power consumption of small devices is usually measured in Watts, and the power consumption of larger devices is measured in kilowatts (kW), or 1,000 Watts.

Electricity generation capacity is often measured in multiples of kilowatts, such as megawatts (MW) and gigawatts (GW). One MW is 1,000 kW, and one GW is 1,000 MW.

Electricity use over time is measured in Watthours

A Watthour (Wh) is equal to the energy of one Watt steadily supplied to, or taken from, an electric circuit for one hour. The amount of electricity that a power plant generates or an electric utility customer uses over a period of time is typically measured in kilowatthours (kWh), which is the number of kilowatts generated or consumed over one hour. For example, if you use a 40-Watt (0.04 kW) light bulb for five hours, you have used 200 Wh, or 0.2 kWh, of electrical energy.

Utility companies measure and monitor electricity use with meters

Electric utilities measure the electricity consumption of their customers with meters that are usually located on the outside of the customer's property where the power line enters the property. In the past, all electricity meters were mechanical devices that a utility employee had to read manually. Eventually, automated reader devices became available. These meters periodically report electricity use to utilities from mechanical meters with an electronic signal. Now, many utilities use electronic smart meters, which provide wireless access to the meter's power usage data, to measure electricity consumption on a real-time basis. Some smart meters can even measure the electricity use of individual devices and allow the utility or customer to control electricity use remotely.