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. A Watt is the unit of electrical power equal to one ampere under a pressure of one volt. It was named to honor James Watt, the inventor of the steam engine.

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

Electricity generation capacity is typically measured in multiples of kW, 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 is equal to the energy of one Watt supplied to, or taken from, an electric circuit steadily for one hour. The amount of electricity a power plant generates or the amount of electricity 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 in one hour.

For example, if you use a 40-Watt (0.04 kilowatt) light bulb for five hours, you have used 200 Watthours, or 0.2 kilowatthours, 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 had to be read manually by a utility employee. Eventually automated reader devices were developed that reported electricity use from mechanical meters with an electronic signal on a periodic basis. Now, electronic smart meters can be used to measure electricity consumption on a real-time basis and can provide access to the data using wireless networks and the Internet. Some smart meters can even measure the use of individual devices and allow the utility or customer to control electricity use remotely.