Consider a load connected to an AC source. The voltage and currents across this load can be represented in the frequency domain.

The complex power is defined as half the product of the voltage and the complex conjugate of the current.

The magnitude of complex power is known as the apparent power.

When expressing complex power in rectangular form, the real part represents the average power, while the imaginary part represents the reactive power.

Complex power can also be expressed in terms of impedance, which is the ratio of voltage to current.

Separating the resistive and the reactance parts of the impedance and comparing these with the obtained complex power expression gives an expression that involves impedance.

The real part of the power is the only useful power delivered to the load.

Reactive power reflects energy exchange between the source and the load. It is zero for resistive, negative for capacitive, and positive for inductive loads.

The complex, real, and reactive power can be visualized in the form of a triangle, known as the power triangle.