... p.d. across a device is V and the current through it is I, the electrical energy W transferred from it in time t is W = ItV The power P of the device will be P = W = ItV t t P = IV The unit of power is the watt (W) and equals an energy transfer rate of 1 joule per second, i.e. 1W = 1J s-1. In the expression P=IV, P will be in watts if I is in amperes and V in volts. A larger unit is the kilowatt (kW) which equals 1000 watts. If all the electrical energy is transformed into heat by the device it is called a 'passive' resistor and the rate of production of heat will also be called IV. The resistance is R. P = IV = V R V² R = I IR ...

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