Imaginary power:

If an alternating current circuit contains reactance, things become interesting. The rate of energy expenditure is the same as VA power in the pure resistance. But when inductance and capacitance exists in an alternating current circuit, these 2 definitions of power part ways. The VA power becomes greater than power manifested as heat, light, radio waves. The extra power is known as imaginary power, as it exists in the reactance, and reactance is, as you have learned, rendered in the mathematically imaginary numerical form. It is known as reactive power.

Inductors and capacitors store energy and release the fraction of a cycle after that. This phenomenon, like the true power, can be expressible as the rate at which energy can be changed from one form to the other. But instead of any immediately usable form of power, like radio or sound waves, imaginary power is stashed as a magnetic or electric field and then dumped back into circuit, again and again.

You can take of the relationship between imaginary and true power in same way as you think of potential versus kinetic energy. A brick held out of a 7th-story window has potential energy, just like a charged-up capacitor or inductor has imaginary power.

Although the label imaginary power carries a connotation which it is not real or important, it is significant indeed. Imaginary power is responsible for many aspects of alternating current circuit behavior.