How To Calculate Kilowatt Hours

A kilowatt-hour is a basic unit of energy that applies especially to electricity. A watt is a volt times an amp, and there are 1000 watts in a kilowatt. A watt is a unit of power, which is the rate of energy used. When you multiply power by time you get a quantity of energy. Energy is measured in many different unit systems in different applications. Your electric utility company bills you for the kilowatt-hours you use. The energy content of batteries is given in amp-hours. The energy density of liquid fules is often given in British Thermal Units per gallon, or BTUs per gallon. A home gas bill usually reports usage in cubic feet (of gas), or therms. A therm is 100 cubic feet, and equivalent to 1000 BTU. Using the right conversion ratios, you can calculate the equivalent number of kilowatt-hours (KWH) in any amount of of energy measured in these other units. You can use the link in the Resources to calculate conversion ratios.

Step 1

Calculate kilowatt-hours in natural gas. To compare the amount you pay for natural gas versus electricity, you need to convert between kilowatt-hours and therms. The conversion ratio is 0.0342 therms / 1 kilowatt-hour = 1. Electricity and natural gas prices vary greatly from region to region, but typical prices are 0.10 dollars per kilowatt-hour and 1 dollar per therm. Calculate (1 dollars / therm)*(0.0342 therm /1 kilowatt-hour) and cancel therms to get 0.0342 dollars/kilowatt-hour. In many places, natural gas is indeed cheaper per unit of energy. This is why, when it is available, many homes use gas for heating and cooking rather than electricity.

Step 2

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Calculate kilowatt-hours in a battery. A battery is usually labeled with how many milliamp-hours it has. Amp-hours are not technically a unit of energy. But when you multiply amp-hours by volts, you get watt-hours. There are 1000 watt-hours in a kilowatt-hour. A typical alkaline 1.5 volt AA battery supplies around 2000 milliamp-hours. If you multiply 2000 milliamp-hours x 1.5 volts you find that an alkaline AA battery contains 3 watt-hours, or 0.003 kilowatt-hours of energy. You can purchase AA batteries for as little as fifty cents apiece, or 0.5 dollars. To find the cost per kilowatt-hour, calculate (0.5 dollars / 3 watt-hours) x (1000 watt-hours / kilowatt-hour) = 167 dollars/kilowatt-hour. You can see why rechargeable batteries are good for the environment and your bank account.

Step 3

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Calculate kilowatt-hours in gasoline. Gasoline contains about 100,000 BTU of energy per gallon. The conversion ratio between BTU and kilowatt-hours is 3412 BTU / 1 kilowatt-hour = 1. So, gasoline contains (100,000 BTU / gallon) x (1 kilowatt-hour / 3412 BTU) = 29.3 kilowatt-hour/gallon. In 2013, the average price of gasoline was around 3.5 dollars/gallon. Calculate ( 3.5 dollars / 1 gallon ) x (1 gallon / 29.3 kilowatt-hours) = 0.12 dollars / kilowatt-hour. Roughly the same as for household electricity.