Watts to Horsepower (mechanical) Converter

The watt is the SI unit of power: one joule of energy transferred each second. Its symbol is W, and it is the base from which every larger power unit is built.

Watt ratings appear on light bulbs, televisions, kettles, solar panels, inverters and small generators. Physics and engineering use the watt for every rate of energy transfer, from a laser to a motor.

Because power is a rate and energy is an amount, a watt multiplied by a duration gives energy in joules or watt-hours. Metric steps above the watt are exact powers of one thousand.

A frequent error treats the watt as a quantity of energy, so an appliance rating is read as consumption. Power needs a time period before it becomes an amount of energy.

Watts suit appliance and electronics figures, kilowatts suit generators, inverters and solar arrays, and megawatts suit grid capacity. Milliwatts cover indicator lights and small sensors. Reading a generator rating in watts rather than kilowatts multiplies the number by a thousand and misstates the scale.

Mechanical horsepower is exactly 745.7 watts, and metric horsepower is exactly 735.49875 watts. The two definitions differ by about 1.4 percent, so a figure carries a larger number in mechanical horsepower.

Vehicle engine output, pump and compressor motors, and agricultural machinery are commonly rated in horsepower. Metric horsepower is used for engine figures in much of Europe and Asia, while mechanical horsepower dominates in the United States.

Power is a rate, so multiplying horsepower by a running time gives an energy amount in joules or watt-hours. The gap of about 1.4 percent between the two horsepower definitions is carried through every conversion to watts.

Comparing two engine ratings without knowing which horsepower definition applies is the usual error, and the numbers are not interchangeable. The definition behind a figure should always be established before it is converted.

Horsepower is the unit for engine output, while kilowatts fit appliances, generators and inverters, and BTU per hour fits heating and cooling equipment. Metric horsepower is a separate sibling, so a PS figure and an hp figure differ even when they describe the same engine.