Horsepower-hour (hp⋅h)

US & ImperialTraditional unit (still used in some regions)

About 2.685 MJ (0.7457 kWh), the energy of one mechanical horsepower for an hour.

1 horsepower-hour = 2,684,519.538 joules (approximately).

About the horsepower-hour

Multiplying a power by a time gives energy, and this unit does it with James Watt's horse. In the late 18th century Watt compared steam engines with the draft horses they replaced. After an experiment in 1782 that suggested a brewery horse delivered 32,400 foot-pounds per minute, he and Matthew Boulton rounded the figure to 33,000 foot-pounds per minute the next year. One mechanical horsepower is therefore about 745.7 W, and sustaining it for an hour gives roughly 2.685 MJ, or 0.7457 kWh.

The unit is now outdated and outside the SI. Railways still use it to settle accounts when locomotives are lent between companies. In one example, a 2,500 hp unit used for 12 hours is owed back as 30,000 hp-hours, repayable with a 3,000 hp unit for 10. A German variant, the Pferdestaerkenstunde, uses the metric horsepower of about 735.5 W and equals 2,647.7955 kJ, so one hp-hour is about 1.014 PSh.

Sources: Wikipedia: Horsepower-hour; Wikipedia: Horsepower; Wikipedia: Conversion of units of energy

Horsepower-hour conversion chart

1 horsepower-hour expressed in every other energy unit:

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