Instantaneous and Average Power

Instantaneous Power

Instantaneous power p(t) absorbed by an element is the product of the instantaneous voltage v(t) across the element and the instantaneous current i(t) through it.

This definition of instantaneous power is valid for signals of any waveform.

From the formula above, this shows us that the instantaneous power has two parts. The first part is constant or time independent. Its value depends on the phase difference between the voltage and current. The second part is a sinusoidal function whose frequency is 2ω, which is twice the angular frequency of the voltage or current.

Average Power

P can be defined in two ways: as the real part of the complex power or as the simple average of the instantaneous power. The second definition is more general because with it we can define the instantaneous power for any signal waveform, not just for sinusoidal.

The instantaneous power changes with time and therefore difficult to measure. The average power is more convenient to measure.

NOTE: A resistive load absorbs power at all times, while a reactive load ( L or C) absorbs zero average power.