#### SI units (International System)

Power is the amount of energy per time unit consumed or transferred by a system. The derived SI unit for power is the watt (W) named after the Scottish engineer James Watt most famous for his improvement on the steam engine. One watt is a joule of energy per second. Equivalent you may say that a watt is the rate of work needed to keep an object moving at one metre per second with an opposition force of one newton. In electromagnetic terms, one watt is the rate at which work is done when one ampere of current through a conductor of electric potential difference of one volt.

#### Common Power Units

There are different colloquial versions of the calorie, see the article on the calorie under the energy section to distinguish them apart. Here, a calorie is about the amount of energy needed to raise the temperature of water one degree Celsius.

The horsepower (hp) was introduced by James Watt to describe how his steam engines compared against the work done by draft horses. The most common definition of horsepower for electric power is 746 watt. We have added some additional common variations of the horsepower in the table.

#### British and American Power Units

The U.S. refrigeration unit (USRT) is often used to measure the power of the heat-extraction capacity of refrigeration and air conditioning installations. A USRT is defined as the heat of fusion absorbed by melting 1 short ton (0.893 long ton or 0.907 metric ton) of ice at 0 °C (32 °F) during 24 hours.

The British Thermal Unit is a unit of energy which is defined in the energy section.