|SI units||2.685 MJ|
|English Engineering Units|
A horsepower-hour (symbol: hp?h) is an outdated unit of energy, not used in the International System of Units. The unit represents an amount of work a horse is supposed capable of delivering during an hour (1 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):
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 hp × 12 h) = 30,000 hp?h. Railroad A may repay the debt by loaning Railroad B a 3,000 horsepower locomotive for ten hours.