What Is a Kilowatt-Hour on Electric Bills
Power Companies Bill Customers for Kilowatt-Hours of Electricity
Feb 16, 2009
When the monthly electric bill comes, the electric power company charges for the number of kilowatt-hours of electricity a customer uses. What exactly is a kilowatt-hour?
Energy and Power
People often talk about electric power and electric energy as if they were the same thing. However power and energy have distinctly different meanings in physics, other sciences, and engineering. To understand what a kilowatt-hour is, one must first understand this difference.
Start with a familiar analogy. Wanda walks three miles in an hour and Ralph runs three miles in a half hour. Both Wanda and Ralph travel the same distance, but Ralph travels at a greater speed, which is the distance divided by the time. Running Ralph covers the same distance in half the time as Walking Wanda, so his speed is twice as much.
Energy and power have the same relationship as distance and speed. The power is the energy divided by the time.
It depends on a person's body weight, but walking briskly and running use approximately the same amount of energy per mile. So Wanda and Ralph each burn about the same amount of energy walking or running three miles. Because Ralph uses the energy in half the time as Wanda his power is twice as much.
Joules, Watts, and Kilowatts
Physicists and other scientists measure energy in joules. A joule is very approximately the amount of energy it takes to lift an apple from the floor to a table. Because power is energy divided by time, power is measured in joules per second. A joule per second is defined as a watt. That means a 100 watt light bulb uses 100 joules of energy every second.
The prefix kilo means 1000, so a kilowatt is 1000 watts. A 100 watt light bulb consumes 0.1 kilowatts of power. Ten 100 watt light bulbs will use a kilowatt of power. Notice that watts and kilowatts already include the time, so saying that it takes an hour (or any other time interval) for a light bulb to burn a kilowatt of electricity is incorrect.
Kilowatt-Hours
A kilowatt-hour is a unit of energy (not power) that is used by electric companies, but seldom by scientists. Kilowatts are power units, and kilowatt-hours are kilowatts multiplied by hours. Power is energy divided by time, so power multiplied by time is energy. Kilowatt-hours is a power unit (kilowatts) multiplied by a time unit (hours), so it is an energy unit. One kilowatt-hour equals 3,600,000 joules.
A 100 watt light bulb uses 0.1 kilowatts of power. Burning the light bulb for 10 hours uses 0.1 kilowatts multiplied by 10 hours, which equals 1 kilowatt-hour of energy. The electric company will then charge for a kilowatt-hour of electricity.
Power, which is energy divided by time, is measured in watts or kilowatts. Physicists and other scientists measure energy in joules, but electric companies measure energy in kilowatt-hours.
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