What are kilowatt-hours (kwh)?
If you look at the bill your family receives from the electric company, in most cases you will see that it is charging you for the total number of “kilowatt-hours” that your family used during the past month. For example, it might say that your family used 1,000 kilowatt-hours, so that if the company charges a price of 15¢ per kilowatt-hour, then your total bill will be 1,000 × 15¢ = $150. But what exactly is a kilowatt-hour?
You can probably figure out that it must be a unit of energy, because the electric company charges for the total amount of energy (not power) that you use. In fact, a kilowatt-hour is defined as the total amount of energy that you consume in 1 hour if you are using 1 kilowatt of power. This definition is easier to see if we write it as an equation:
We can simplify this further by remembering that 1 kilowatt is 1,000 joules per second and 1 hour is the same as 3,600 seconds (because there are 60 seconds in one minute and 60 minutes in one hour, and 60 × 60 = 3,600). If we put those values in the above equation, we find:
In other words, a kilowatt-hour is just another name for 3.6 million joules of energy.
In case you are wondering why electric companies bill in kilowatt-hours rather than joules, it is because it makes it easier to think about your home energy consumption. For example, suppose you have ten light bulbs turned on, and each one is a 100-watt light bulb. Then the ten light bulbs combined are using 1 kilowatt (1,000 watts) of power, which means they use 1 kilowatt-hour of energy if you leave them turned on for an hour.
By the way, another place you’ll see kilowatt-hours is if you go for a ride in an electric car. For example, if the car has a 100 kilowatt-hour (100 kwh) battery, it means the battery can provide 100 kilowatt-hours of energy before it must be charged again. A typical electric car can go an average of about 5 kilometers with 1 kilowatt-hour of energy, so a 100 kwh allows the car to drive about 500 kilometers before running out of charge.