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Convert HP into kW and vice-versa
Wood-Mizer sawmills can be fitted with electric, gasoline or diesel main engines. Sometimes it is necessary to convert horsepower into kW or vice-versa. You can use our calculator below for the conversion.
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'Horsepower' (HP) describes several units of measurement of power. In our calculator we use imperial horsepower. 550 foot-pounds per second is roughly equivalent to 745.7 watts.
Generally, HP is equivalent to 735.5-750 watts, usually averaged as 746 watts. In the early 18th century, during Great Britain's industrial revolution, horsepower needed to be defined to compare the output of steam engines with the power of the shire horses which powered and pulled machinery - 33,000 foot-pounds of work per minute, would you believe?
Subsequently, horsepower became the comparative measurement of the output of piston engines, turbines, electric motors and other machinery.
Where horsepower came from
The development of the steam engine led to a comparison of the output of horses with that of the engines. In 1702, the inventor Thomas Savery wrote (in what is today garbled English) in The Miner’s Friend: "So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work…"
The idea was later used by the Scotsman, James Watt to help market his improved steam engine. Watt calculated that a horse could turn a mill wheel 144 revolutions an hour (2.4 each minute). The wheel was 12 feet in radius so the poor horse travelled 2.4 × 2π × 12 feet in one minute. Watt discovered that the creature could pull with a force of 180 pounds and he rounded it to an even 33,000 ft·lbf/min.
He also worked out that a pony could lift an average 220 lbf (0.98 kN) 100 ft (30 m) per minute in a four-hour shift. Next he adjudged a horse was 50% more powerful than a pony and thus arrived at the 33,000 ft·lbf/min figure.
At first, the English civil engineer, John Smeaton worked out that a horse could produce 22,916 foot-pounds per minute but John Desaguliers increased that to 27,500 foot-pounds/min. In turn, James Watt, in 1782 found that a brewery horse could produce 32,400 foot-pounds per minute. In 1783 James Watt and his fellow engineer Matthew Boulton standardized it to 33,000.
Even so, horsey people who understand their animals' capabilities claim that Watt was either a optimistic or wanted to under-promise and over-deliver. Certainly, few horses can maintain the effort for long.
Nevertheless, the comparison with a horse became the yardstick.
Fit human beings can deliver approximately 1.2 hp briefly and keep up 0.1 hp indefinitely; athletes manage up to about 2.5 hp briefly and 0.3 hp sustainably.
Metric horsepower
Metric horsepower, as a rule, is defined as 0.73549875 kW, or roughly 98.6% of mechanical horsepower. Obviously, when measurement systems varied considerably and engines delivered less thrust than today this was not at the top of technicians' agendas but today it is very important. European 'Trophy' vehicles such as the McLaren F1 and Bugatti Vevron are often quoted using incorrect definitions, their output being sometimes converted as much as twice through confusion over whether the horsepower figure from the start was metric or mechanical.
Horsepower definitions
Defining horsepower is difficult, because it varies between applications:
- Mechanical horsepower AKA imperial horsepower of exactly 550 foot-pounds per second is roughly equivalent to 745.7 watts.
- Metric horsepower, 75 kgf-m per second is equivalent to about 735.499 watts.
- Boiler horsepower, for rating steam boilers translates to 34.5 pounds (≈15.65 kilograms) of water evaporated hourly at 212 degrees Fahrenheit (=100 degrees Celcius, =373.15 Kelvin), or 9,809.5 watts.
- An imperial horsepower for rating electric motors is the same as 746 watts.
- Outside the UK, on the European continent electric motors formerly had dual ratings, based on 0.735 kW for 1 HP
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