Electric power is the rate at which energy is transferred by a circuit. The symbol for power is P and it is measured in watt (W). The Watt is defined as one joule per second. Electric power is equal to the voltage multiplied by the current.


The electric power can be calculated with one of the following formulas.

P is the symbol for power and is measured in watt (W).
V is the symbol for voltage and is measured in volt (V).
I is the symbol for current and is measured in ampere (A).

AC power

AC power

In an AC circuit, the voltage and current are varying, so the power can't be calculated with P = V·I. The (active) power can be calculated by multiplying the right side of the formula by cos(φ). cos(φ) is called the power factor and φ is the phase. The right side has to be multiplied with when three phase power is used instead of one phase.