A kWh, or kilowatt-hour, is a measure of electricity usage. Most utilities base their billing on the number of kWh consumed during the billing period.
Understanding kWh calculations allows you to control your electric bill. You can calculate the cost of operating any electrical device.
One kilowatt is 1,000 watts. Running a device that consumes 1,000 watts for one hour results in the consumption of one kilowatt-hour, or kWh, of electricity. By the same token, one hour of running several devices whose combined power use is 1,000 watts--for example, 10 100-watt light bulbs--also consumes one kWh.
Calculating Power Consumption
Each type of electrical device in your home is rated for power consumption by the manufacturer. A compact fluorescent light bulb might, for example, be rated at 13 watts, while an electric range might be 10,000 watts. You can find this information on the packaging, in the owner's manual or printed on the device label. Dividing the wattage rating by 1,000 gives the power consumption in kilowatts. You then multiply this value by the number of hours of operation to obtain the number of kWh consumed.
You want to find out how much it costs per month to operate your outdoor security lighting for eight hours each night. You have four lighting units, each of which has a 75-watt bulb. A kWh costs 42 cents in your area. Four bulbs at 75 watts each equals 300 watts, or 0.3 kilowatts. Multiplying by eight hours gives 2.4 kWh per night. For a 30-day month, this equals 72 kWh. At 42 cents per kWh, your monthly cost equals $30.24, or just over a dollar a day.
Manufacturers advertise the cost-savings from using compact fluorescent lamps (CFLs) by basing their claims on the kWh these lamps consume as compared to incandescent lamps of similar brightness. Because CFLs produce more light with less electricity, operating them for a given period consumes fewer kWh and therefore saves money.