Energy, Power and Torque Converter

Three quantities that are easy to mix up

This page groups three related quantities. They cannot be converted into one another, only within themselves, so the converter asks which one you mean first.

Energy is an amount: how much work is done or heat is transferred. The SI unit is the joule. The calorie comes in two sizes that differ by a factor of 1,000: the small calorie (cal) of chemistry, exactly 4.184 joules on the thermochemical definition, and the kilocalorie (kcal), which is the “Calorie” on food labels. The kilowatt-hour on an electricity bill is also energy: exactly 3.6 megajoules.

Power is a rate: energy per second. One watt is one joule per second, a kilowatt is 1,000 watts and a mechanical horsepower is about 745.7 watts. To get from power to energy you need a time. A 2 kW heater running for 3 hours uses 6 kWh.

Torque is a turning force: force times lever arm. Its SI unit is the newton-meter. The foot-pound used on American torque wrenches is 1.3558 N·m. A newton-meter of torque has the same dimensions as a joule but is a different thing, which is why torque is never written in joules.

Electrical quantities need more than a factor. Volts become watts only once you know the current, which is what the volts to watts calculator is for. And US and European mains voltage are two different supply standards, not two units: the US to Europe voltage checker tells you whether a device can be plugged in abroad.