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**Power** is the rate at which energy is transferred, consumed or transformed in a unit of time. Therefore the time it takes to do work is important. For instance, if you do little work in a short period of time, you have more power than if you do more work over a long stretch of time.

The base unit of power is the watt, which is equal to one joule per second. The unit was named after James Watt, the inventor of the stream engine. The horse power, which is the equivalent of 750 watts, is also occasionally used with machinery.