What is the difference between a watt and horsepower?

Answer 1

Both are used to measure power. The horsepower is simply the Imperial unit for power, whereas the watt is the SI unit for power, where one horsepower is equivalent to 746 watts.

In North America, it is traditional to measure the output power of an electric motor in horsepower, while the watt is used to measure its input power. Elsewhere in the world, the watt is used to measure both input power and output power. (The difference between output power and the input power is due to losses, such as heat transfer, friction, windage [air resistance], etc.)

Outside North America, the output power of a car engine is now measured in kilowatts rather than in horsepower.