A Horsepower-hour (hph) is an outdated unit of energy, not used in the SI system of units.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):
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.[1]