Power Converter

Ten units of power in one grid: milliwatt, watt, kilowatt, megawatt, mechanical horsepower, metric horsepower, BTU per hour, foot-pound per second, calorie per second and kilocalorie per hour. Everything is a pure multiplication through the watt, and the definitions are the exact ones rather than rounded textbook figures.

The reason to be careful is that three of these units come in more than one flavour. Mechanical horsepower is 550 ft·lbf/s, which works out at 745.6998715822702 W, while metric horsepower — PS in German, cv in French and Italian — is 75 kgf·m/s, exactly 735.49875 W. They differ by 1.39%, so a car advertised at 100 PS is 98.6 hp, and a spec sheet that does not say which one it means is ambiguous by more than the difference between two model years.

The BTU row is per hour, which is the trap in air conditioning. A unit sold as "12,000 BTU" is rated at 12,000 BTU per hour, and that is 3517 W — the figure also known as one ton of refrigeration, because it is roughly the rate at which a ton of ice melts over a day. Reading the number as energy rather than as a rate puts you out by a factor of 3600.

The calorie is the third. This tool uses the thermochemical calorie of exactly 4.184 J, which is the one chemistry uses; the International Table calorie is 4.1868 J, a further 0.067% away. And the Calorie on food packaging is a kilocalorie, a thousand of these, so the kcal/h row rather than the cal/s row is the one that lines up with a nutrition label — 100 kcal/h is 116.2 W, which is about what a person at rest gives off.