Watts are a measurement of how many joules of work can be done in one second and are commonly used to indicate how much power an electrical device uses. Kilowatt hours are a measurement of energy and are used to calculate how much work can be done in an hour with one kilowatt--1,000 watts--of power. To determine how much energy an electronic device uses over a period of time, you must convert watts into kilowatt hours.
Determine how many watts are spent in an hour. If you have a 60W light bulb, for every hour the bulb is turned on, it uses 60W of power.
Divide the number of watts by 1000 to convert the number from watts to kilowatts. 60W divided by 1000 results in 0.06kW.
Multiply the number of kilowatts by the number of hours the device will be used. If the device will be used for three hours, multiply 0.06kW by three to produce 0.18kWh. If a 60W light bulb is left on for three hours, it will use 0.18kWh of energy.