True power is the power manifested in tangible form such as electromagnetic radiation, acoustic waves, or mechanical phenomena. In a direct current (DC) circuit, orin an alternating current (AC) circuit whose impedance is a pure resistance, the voltage and current are in phase. P is the power in watts, Erms is the root-mean-square voltage in volts, and Irms is therms 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 Erms. Irms is larger than the true power, and is known as apparent power. The vector difference between the apparent and true power is called reactive power, and represents energy alternately stored and released by inductor and/or capacitor.