Watt performed some reverse engineering to determine how many horses would equate to a steam engine in order to communicate the cost efficiency of his product. Watt's horsepower (hp) unit of measurement is based on a horse pulling a twelve-foot mill wheel for four hours. Using the this formula, the maximum output of a horse is around 15 horsepower per day. By 1969, 1200 steam engines had been sold, totalling 118,500 horsepower (approx 99 hp each).
Therefore, each steam engine replaced about 6.6 horses on average per day.