How this calculator works
Power is the rate at which energy is transferred, so one watt is one joule per second. Every unit here is converted through the watt. Horsepower is not a single unit: mechanical horsepower is 550 foot-pounds per second, metric horsepower is 75 kilogram-force metres per second, electrical horsepower is exactly 746 W, and boiler horsepower describes steam-raising capacity. A ton of refrigeration is 12,000 Btu per hour, the rate needed to freeze one short ton of ice per day.
Formula
watts = entered value x F(unit). 1 kW = 1,000 W and each SI prefix is a power of ten. 1 mechanical hp = 745.699871582 W. 1 metric hp = 735.49875 W. 1 electrical hp = 746 W. 1 boiler hp = 9,809.5 W. 1 Btu/h = 0.2930710702 W. 1 kcal/h = 1.162222222 W. 1 erg/s = 10^-7 W. 1 ton of refrigeration = 12,000 Btu/h = 3,516.852842 W.
Worked example
A 2 kW kettle is 2,000 W, 2.682 mechanical hp and 6,824 Btu/h. A 100 W bulb is 0.1 kW, 0.134 hp and 86.04 kcal/h. One ton of refrigeration is 3.517 kW.
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Frequently asked questions
Why are there several kinds of horsepower?
They arose in different industries. Mechanical horsepower comes from James Watt's estimate for a horse lifting coal; metric horsepower uses a round kilogram-force value; electrical horsepower was fixed at 746 W; boiler horsepower measures steam output rather than shaft power.
Is a ton of refrigeration a mass?
No. It is a cooling rate equal to 12,000 Btu per hour, roughly 3.517 kW. The name comes from the heat needed to melt one short ton of ice in a day.
Which horsepower does a car advertisement use?
North American car power is usually mechanical horsepower. European figures are often metric horsepower, written PS or CV, which is about 1.4 per cent smaller.