Power Converter

Power is the rate of doing work — energy per unit of time — which is why watts and horsepower measure the same thing despite feeling like they belong to different worlds. A watt is one joule per second, and everything else here is a multiple of it.

There are two horsepowers and they differ by about 1.4%, which sounds trivial and is not when comparing car figures. Mechanical horsepower is 745.7 W and is what British and American figures use. Metric horsepower — written PS in Germany or CV in France and Italy — is 735.5 W, defined as the power to lift 75 kg by one metre in one second. A car quoted at 200 PS is about 197 hp, so the German figure always looks slightly better.

BTU per hour is the unit air conditioning and heating are sold in, particularly in North America. A 12,000 BTU/h unit — often called a one-ton unit, from the rate at which it would melt a ton of ice in a day — is about 3.5 kW.

One practical note about electrical appliances: the wattage on a label is the power it draws, and multiplying by hours gives the energy in watt-hours, which is what the bill is based on. A 2 kW heater run for three hours uses 6 kWh regardless of how warm the room gets.