When James Watt invented his steam engine, he was asked how many horses the engine could replace. To find out, Watt built a rope-and-pulley device with which he could measure the time it took a horse to lift various weights. After many experiments Watt concluded that a horse could produce 746 Watts of power for a sustained period of time. (Watt used English units of measurement in his experiments. The metric unit of power was later named in his honor). Watt's definition of horsepower is still used today, even though the power output of an average horse during the course of a working day is only about two-thirds of a standard horsepower