What is horsepower?

9 December 1911

HAVE YOU EVER pondered the origin of the unit horsepower? It’s been used since before even the advent of the car but never fully understood by many. So, in 1911, Autocar explained to our readers.

It was devised by James Watt in 1872 as a tool for selling steam engines to replace the horses that were used to turn the wheels for pumping water out of mines.

Seeing that a horse expending above-average effort could turn a 12ft-radius wheel 144 times an hour with an estimated force of 180lb ft, Watt calculated one horsepower to be 33,000lb ft per minute.

Brake horsepower, later devised for combustion engines, accounts for power expended in the pumping and frictional losses of driving the engine itself – only one or two horses, depending on efficiency.

Lotus’s new hypercar (p8), then, is flexing its water-pumping ability.