A kilowatt hour is equal to 1000 watts for one hour. To calculate kilowatt hours a particular device in your home is using, find the wattage of the device, multiply it by the number of hours you used it, and divide by 1000. You can also use a handy consumption calculator to do it for you. Once you have that number, you can multiply it by the energy charge rate for your schedule of service to determine how much you are spending for that particular device.
This is particularly useful in selecting lighting. A 100 watt incandescent bulb will cost you four times as many kWh as a 25 watt bulb, but it shines much brighter. To get the same brightness as a 100 watt bulb, a compact fluorescent will need only 23 watts and an LED, only 14-20 watts.