Understand Your Electricity: What is a kilowatt hour (kWh)

What is a kilowatt hour (kWh)?

Glad you asked. We measure the electricity we use at any given moment by watts: a 100 watt bulb is using 100 watts continuously while it is on.

We buy our electricity by the Kilowatt hour or kWh. A kilowatt hour is not a period of time but an amount of power. The dictionary definition of kWh is: a unit of energy equal to the work done by one kilowatt in one hour.

I find this a bit abstract; it is easier to make sense of via equivalences. A device's wattage determines how long it must run before it uses up 1 kWh's worth of power.

1 kWh = the amount of power a 100 watt device uses in 10 hours
1 kWh = the amount of power a 10 watt device uses in 100 hours
1 kWh = the amount of power a 1 watt device uses in 1000 hours

From another perspective:
1000 ÷ wattage = the number of hours the device must run to equal 1 kWh

To calculate how many kWh a device uses in a year, you must know its wattage and how many hours a day it is running.

For a 50 watt device that runs 4 hours a day:
1000 ÷ 50 = 20 hours to make 1 kWh
4 X 365 = 1460 = the hours the device runs in a year
1460 ÷ 20 = 73 kWh per year

As usual, Mr. Electricity has a helpful discussion of the wonders of the watt and kilowatt hour.