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.

BTU per hour (BTU/h) equals approximately 0.293 watts, the standard rating unit for air conditioners, heaters, and furnaces in North America. A typical window air conditioner rated at 10,000 BTU/h can cool a 400–500 square foot room, making this unit essential for HVAC system sizing. One BTU/h equals about 0.2931 watts.