Horsepower-hour (hph)

The total work performed by one mechanical horsepower sustained for a full hour.

James Watt faced a commercial problem rather than a scientific one: mine owners understood ponies, not engines. He measured the sustained work a pit horse could perform and set one horsepower at 33,000 foot-pounds per minute, a figure pitched deliberately high so that no customer could claim his machine underperformed the animals it displaced. The horsepower-hour then became the natural unit for expressing total engine output across a working shift.