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 = ErmsIrms. In this formula, P is the power in watts, Erms is the rms voltage in volts, and Irms is the rms current in amperes. However, in an AC circuit, impedance consists of reactance as well as resistance. As a result, the voltage and current are not in phase, which complicates the determination of power.
Apparent power is the total power that is delivered to the circuit, and it is the sum of the real power and reactive power. The combination of reactive power and true power is called apparent power, and it is the product of a circuits voltage and current, without reference to phase angle. Apparent power is measured in VA, the product of volts and amperes. In a purely resistive load, where the current and voltage are in phase, the apparent power is equal to the real power. However, in an inductive or capacitive load, where the current and voltage are not in phase, the apparent power is greater than the real power due to the presence of reactive power.
