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.

Calories per second (cal/s) equals exactly 4.184 watts, used primarily in thermodynamics and physics to express heat transfer rates. While not a common everyday unit, it bridges thermal and mechanical power calculations in scientific research. One calorie per second equals exactly 4.184 watts or about 0.00561 horsepower.