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*_{rms}*I*_{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 the true 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 *P*_{a} represents the apparent power in a complex AC circuit, *P*_{t}
represents the true power, and *P*_{r} represents the reactive power, then
the following equation holds:

*P*_{a}^{2} = *P*_{t}^{2}
+ *P*_{r}^{2}

*This was last updated in*March 2010

*Posted by:*Margaret Rouse

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