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.

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.