Have you ever wondered how your electric bill is calculated? You know that you are using 60-watt bulbs throughout your home, for example, but you may not have known that if one of those bulbs is turned on for five hours a day, it uses 10.8 kilowatt hours per month--at a cost of about 80p per bulb, based on the national average. Multiply that by all the bulbs in your home and you begin to see how your power bill can add up. Electricity can be measured in wattage, or the rate at which power is used at a given moment. A thousand watts is equal to 1 kilowatt (kW). Electric companies charge by the kilowatt hour (kWh), which is simply how many thousands of watts you use each hour. Starting with an object's wattage or kilowattage, you can calculate kilowatt hours using simple multiplication.

Find out how many watts a device uses per hour. Look on the label or in the instruction manual, or use Mr. Electricity's interactive chart at michaelbluejay.com (see Resources for link). A 60W light bulb obviously uses 60 watts per hour. An average laptop uses about 45 watts per hour.

Multiply the wattage by .001 to get the kilowattage (kW). A 60W light bulb uses 0.06kW per hour, for example.

Multiply the kW by the number of hours of usage to get the kW hours (kWh). A 0.06kW light bulb uses 1.44kWh in 24 hours.