True power is the power manifested in tangible form such as electromagnetic radiation, acoustic waves, or mechanical phenomena. In a direct current ( DC ) circuit, or in an alternating current ( AC ) circuit whose impedance is a pure resistance , the voltage and current are in phase, and the following formula holds:
P = E rms I rms
where P is the power in watts, E rms is the root-mean-square voltage in volts, and I rms 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, this voltage-current product represents true power . But when there is reactance in an AC circuit, the product E rms I rms is larger than the true power, and is known as apparent power .