Horsepower is a unit of power, while kilowatt-hours is a unit of energy. To go from horsepower to kilowatt-hours, you need to know how long the power is being exerted. For example, a 100-horsepower engine running for five minutes would use significantly fewer kilowatt-hours than the same engine running for five hours.
Multiply the number of horsepower by the time the power is exerted. For example, if you have a 20-horsepower motor running for three hours, you would multiply 20 by 3 to get 60 horsepower-hours.
Multiply the number of horsepower-hours by 0.7457 kilowatts per horsepower-hour to convert to kilowatt-hours. In this example, you would multiply 60 by 0.7457 to get 44.742 kilowatt-hours.
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Check your answer using an online calculator. Enter the horsepower and the appropriate amount of time, and the number of kilowatt-hours will be displayed.