Power is the time rate of expending or absorbing energy, measured in watts (W). Power, is commonly denoted by the letter p or P.

Mathematically,

where p is power in watts (W), w is energy in joules (J), and t is time in seconds (s). From voltage and current equations, it follows that;

Thus, if the magnitude of current I and voltage are given, then power can be evaluated as the product of the two quantities and is measured in watts (W).

Sign of power:

Plus sign: Power is absorbed by the element. (Resistor, Inductor)

Minus sign: Power is supplied by the element. (Battery, Generator)

Passive sign convention:

 If the current enters through the positive polarity of the voltage, p = +vi If the current enters through the negative polarity of the voltage, p = – vi

Polarities for Power using passive sign convention

(a) Absorbing Power (b) Supplying Power