Power

Power is the rate at which energy is transferred or converted. The SI unit is the watt (W), equal to one joule per second, named for James Watt and the steam-engine improvements that made 18th-century industry possible. Watt did not just improve the engine — he invented the marketing. Around 1782, while trying to sell his steam engines to London brewers who relied on draft horses, he needed a unit that brewers could relate to. He measured the work a strong brewery horse could sustain over a shift and rounded it upward to get a comfortable margin: 1 mechanical horsepower equals 550 foot-pounds-force per second, which computes to exactly 745.69987158227022 W.

Continental Europe later adopted a slightly different metric horsepower (PS in Germany, ch in France, cv in Italy and Spain) equal to 75 kgf·m/s or exactly 735.49875 W. This is why the same engine often reads a few units higher in imperial hp than in metric PS on identical spec sheets — a 200 kW motor is 268 hp mechanical but 272 PS metric. European automakers quote PS domestically and hp for export to English-speaking markets.

HVAC work lives in BTU per hour. One ton of refrigeration equals 12,000 BTU/h, or about 3.517 kW — a definition originally tied to the rate at which a ton of ice absorbs heat as it melts. A typical residential air conditioner delivers 2 to 5 tons; commercial rooftop units run 10 to 25 tons each. Data centres and utilities step up through kilowatts, megawatts, and gigawatts. A 1 GW power station covers rough daily consumption of about 750,000 US homes.

Kilocalories per hour appear in heating, metabolism, and nutrition. An adult at rest burns roughly 70 kcal/h (≈ 81 W). A cyclist in a hard effort sustains 250 to 400 W. An Olympic sprinter produces peak instantaneous power above 2 kW.

This converter supports 10 power units — W, mW, kW, MW, GW, horsepower (mechanical), metric horsepower, BTU/h, kcal/h, and ft·lbf/s — with exact defining relationships. Motor ratings, cooling loads, photovoltaic yields, and metabolic-equivalent calculations convert consistently across all of them.