The watt (W) is the SI base unit of power, defined as one joule per second, measuring the rate of energy transfer. From light bulb brightness ratings to phone charger output, the watt appears in everyday product specifications worldwide. One watt equals exactly 1 joule per second, or about 0.001341 horsepower.

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.