For a circuit with a sinusoidal input, the power delivered to a passive element equals the product of the voltage and current across it.

Expressing voltage and current in a sinusoidal form and applying the trigonometric identity gives the expression for instantaneous power with two terms.

The first term is a constant dependent on the phase difference between the voltage and current, while the second term is a sinusoidal function with twice the frequency of the voltage or current.

For circuits with mixed resistance and reactance, the instantaneous power oscillates between positive and negative values.

In a purely resistive circuit, the voltage and current are in phase, resulting in a consistently positive instantaneous power dissipation.

For purely inductive and capacitive circuits, the phase shift between the voltage and current causes the instantaneous power to fluctuate between positive and negative values.

Inductors and capacitors cyclically store and release energy as the current through the inductor or voltage across the capacitor changes.

During the energy release cycle, the reactive elements act as a power source, supplying energy back to the circuit, leading to negative instantaneous power.