Apparent power is a measure of alternating current (AC) power that is computed by multiplying the root-mean-square (rms) current by the root-mean-square voltage. In a direct current (DC) circuit, or in an AC circuit whose impedance is a pure resistance, the voltage and current are in phase, and the following formula holds: P = E_rmsI_rms where P is the power in watts, E_rms is the root-mean-square (rms) 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 an AC circuit, the product of the rms voltage and the rms current is called apparent power. When the impedance is a pure resistance, the apparent power is the same as thetrue power. But when reactance exists, the apparent power is greater than the true power. The vector difference between the apparent and true power is called reactive power. If Pa represents the apparent power in a complex AC circuit, Pt represents the true power, and Pr represents the reactive power, then the following equation holds: Pa2 = Pt2 + Pr2