A Horsepower-hour (hph) is an outdated unit of energy, not used in the SI system of units. The unit represents an amount of work a horse is supposed capable of delivering during an hour (a horsepower integrated over a time interval of an hour). Based on differences in the definition of what constitutes the 'power of a horse', a horsepower-hour differs slightly from the German "Pferdestärkenstunde" (PSh):
- 1.014 PSh = 1 hph = 1,980,000 lb·ft = 0.7457 kWh.
- 1 PSh = 0.73549875 kWh = 2647.7955 kJ (exactly by definition)
The horsepower-hour is still used in the railroad industry when sharing motive power (locomotives.) For example, if railroad A borrows a 2,500 horsepower locomotive from Railroad B and operates it for twelve hours, Railroad A owes a debt of (2,500 * 12) 30,000 horsepower-hours. Railroad A may repay the debt by loaning Railroad B a 3,000 horsepower locomotive for ten hours.
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