In what units is an engine's rated power output usually specified?

The rated power output of an engine is commonly specified in horsepower (HP) or brake horsepower (BHP) because these units reflect the engine’s ability to perform work over time. Horsepower is a measure of the rate at which work is done and is a standard measure used in the automotive and aviation industries to indicate the power available to drive a vehicle or maintain operational capabilities. Brake horsepower specifically measures the output of an engine before losses such as friction are accounted for, providing a more accurate representation of the power actually delivered to the propeller or wheels.

Other options include units that are not appropriate for measuring engine power. Liters or gallons pertain to the volume of fuel or engine displacement, while pounds or kilograms relate to weight, and decibels or hertz are measurements for sound and frequency, respectively.