Horsepower and torque: two different measurements

Torque is a turning effect

A force applied with a perpendicular lever arm from a shaft’s center creates torque. Engine specifications often express it in newton-meters, written N·m, or pound-feet. A longer lever needs less force to make the same torque. This lever example explains a turning effect; it does not describe the full behavior of an engine.

A transmission changes the torque and speed available at the wheels. That is why an engine’s quoted torque is not the same thing as the turning effect at a driven wheel.

Sources: NASA Glenn Research Center ↗ · U.S. Environmental Protection Agency ↗

Power adds the rate of rotation

For a rotating shaft, power depends on torque and rotational speed together. Engine speed is commonly stated in revolutions per minute, or rpm. A specification such as “150 kW at 6,000 rpm” states the power produced at that engine speed. It does not promise the same power at every speed.

A kilowatt, written kW, equals 1,000 watts. One mechanical horsepower is about 746 watts. Metric horsepower, sometimes labeled PS, is about 735 watts. Check which unit a listing uses before comparing numbers across markets.

Sources: Tom Weideman ↗ · NASA Glenn Research Center ↗ · National Institute of Standards and Technology ↗

Common misunderstandings

Torque and power are related, but they are not interchangeable. You cannot directly compare 250 N·m with 150 kW because they measure different quantities.

Do not add a hybrid’s separate engine and motor peak ratings unless the manufacturer identifies that sum as the system output. Their peaks may occur at different operating points. Compare the stated combined rating.