A horsepower-hour (symbol: hp⋅h) is an outdated unit of energy, not used in the International System of Units.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).
Railroad A may repay the debt by loaning Railroad B a 3,000 horsepower locomotive for ten hours.
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