Power Converter

Two different horsepowers

Mechanical horsepower — the one used in the US and UK — is defined as 550 foot-pounds per second, which works out to 745.6999 watts. Metric horsepower, written PS in German, CV in French and Italian, is the power to lift 75 kilograms one metre per second: exactly 735.49875 watts. That is a 1.4% difference, which is why the same car is advertised as 197 hp in one market and 200 PS in another. It is the same engine.

James Watt derived the original figure from what a brewery horse could sustain, deliberately pitching it high so his steam engines would look conservative by comparison. Every horsepower figure quoted since traces back to that marketing decision.

Air conditioners and the refrigeration ton

One ton of refrigeration is the cooling rate needed to freeze one short ton of water in 24 hours: 12,000 BTU/h, or about 3.517 kW. It is a nineteenth-century unit from the days when cooling was sold as delivered ice, and it is still how commercial HVAC capacity is specified. A “3-ton” home system is a 10.5 kW cooling unit, not something that weighs three tons.

Power versus energy, stated plainly

Power is the rate; energy is the total. A 2 kW kettle draws 2 kW the whole time it is on, but uses 0.033 kWh if it runs for a minute. Appliance labels mix the two constantly — a washing machine is rated in watts but its annual running cost is in kWh — so always check whether a figure has a time unit attached.

Frequently Asked Questions