P = ErmsIrms
where P is the power in watts, Erms is the root-mean-square (rms) voltage in volts, and Irms is the rms current in amperes. But in an AC circuit whose impedance consists of reactance as well as resistance, the voltage and current are not in phase. This complicates the determination of power.
In the absence of reactance, the product ErmsIrms represents true power because it is manifested in tangible form (radiation, dissipation, and/or mechanical motion). But when there is reactance in an AC circuit, the product ErmsIrms is greater than the true power. The excess is called reactive power, and represents energy alternately stored and released by inductors and/or capacitors. The vector sum of the true and reactive power is known as apparent power.