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Power is the rate of energy transfer. Since there are several forms of energy, there are several ways of describing power. In general terms of energy, power is defined as
The energy transfer in mechanical systems where work is done by an applied force
Using the relation between work and force
and then differentiating to get power,
The corresponding form of power in rotation is
where  is the torque and
 the angular velocity vector.
Since energy is transfering from a device storing electrical energy to another device in the circuit that converts to another form of energy, power is the rate of change of electrical potential energy. For a DC circuit
The SI unit of the power is one joule per second, which is called watt:
The watt is equal to
 in the base units.
The english units of power are
| 1 joule/second |
= 1 watt |
| 1,000 watts |
= 1 kilowatt |
| 746 watts |
= 1 horsepower |
| 550 ft-lb/sec |
= 1 horsepower |
| 33,000 ft-lb/min |
= 1 horsepower |
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- Frye, Royal M., Applied Physics. Prentice-Hall, Inc., New York, 1947.
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![$\displaystyle 1 \left [horsepower \right] = 1 \left [hp \right] = 550 \left [\frac{ft \, lb}{s} \right]$](http://images.planetphysics.org/cache/objects/235/l2h/img11.png "$\displaystyle 1 \left [horsepower \right] = 1 \left [hp \right] = 550 \left [\frac{ft \, lb}{s} \right]$")