What is "horsepower-hour"

n. a unit of work equal to the work done by one horsepower in one hour

A horsepower-hour (hph) is an outdated unit of energy, not used in the SI system of units.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 (1 (one) 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.