HuntsvilleEngineers mark

Power Converter

W, kW, both horsepowers, BTU/h, and tons of refrigeration.

Input1 hp = 745.6999 W 1 hp(metric)/PS = 735.4988 W 1 TR = 3516.853 W

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The engineering

Motors and engines are the horsepower heartland — but there are two horsepowers. Mechanical (US/UK) hp is 550 ft·lbf/s = 745.7 W; metric horsepower (PS, cv, ch) is 75 kgf·m/s = 735.5 W, about 1.4% smaller. European car brochures historically quoted PS, US ones hp, and dyno numbers cross the Atlantic wearing the wrong hat constantly. HVAC adds BTU/h for heat flow and the ton of refrigeration (12,000 BTU/h ≈ 3.517 kW) for chillers.

The classic mistakes: treating PS and hp as identical (fine for bar conversation, wrong for compliance paperwork), and confusing a chiller's cooling tons with its electrical draw — a 100-ton chiller removes 352 kW of heat while consuming perhaps 60 kW; dividing the two is the COP, not a conversion error, and mixing them up doubles your electrical service by accident.

Where this math comes from

James Watt invented the horsepower around 1783 as marketing arithmetic: to sell steam engines against horses, he rated a dray horse at 33,000 foot-pounds per minute — deliberately generous, so no customer could complain the engine underperformed the animal it replaced. The metric countries later rebuilt the same idea from round metric numbers (75 kgf·m/s), which is why the two horsepowers differ by 1.4% forever.

The watt itself was named in 1889 by the British Association and absorbed into the SI, closing the loop: the man who defined power in horses became the unit that replaced them. The ton of refrigeration is pure American ice-trade archaeology — the heat to melt one short ton of ice in 24 hours, standardized around 1903 as mechanical refrigeration displaced ice delivery, and still on every chiller nameplate a century after the last ice wagon.

  1. 1783James WattDefines the horsepower to sell steam engines against horses.
  2. 1889British AssociationThe watt named as the unit of power.
  3. 1903US refrigeration industryTon of refrigeration standardized from melting ice.
  4. 196011th CGPMThe watt enshrined in the new SI.

See the full timeline of the math behind every calculator →

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