Horses can usually achieve around 14.9 horsepower when at peak physical condition and energy, although this depends on the breed. The horsepower unit measures a steady output of power.
James Watt popularized the concept of mechanical horsepower in the late 18th century to market and sell his steam engine. He estimated that a horse could turn a mill wheel 144 times per hour. Since the wheel's radius was 12 feet, and a horse could pull with a force of 180 pounds, 1 horsepower was said to be 33,000 foot pounds per minute. Some horse experts, however, claim that most horses could not maintain that amount of force over a long work period.