Horsepower (hp) is an imperial unit of power equal to approximately 745.7 watts, originally defined by James Watt to compare steam engine output to draft horses. It remains the standard unit for rating car engines, lawnmowers, and boat motors, especially in the United States. One horsepower equals about 0.7457 kilowatts or 1.0139 metric horsepower.

Metric horsepower (PS, Pferdestärke) equals approximately 735.5 watts, slightly less than imperial horsepower, and is the standard power unit for vehicle ratings throughout Europe and Asia. Car specifications in Germany, Japan, and China typically list engine output in PS. One PS equals about 0.9863 imperial horsepower or 0.7355 kilowatts.