Instantaneous power:

Usually, engineers think of power based on RMS, or effective, alternating current value. But for VA power, peak values are at times used instead. If the alternating current is a sine wave, the peak current is 1.414 times the value of RMS current, and the peak voltage is 1.414 times RMS voltage. If the current and the voltage are in phase, the product of the peak values is twice the product of RMS values.

There are instants in time when VA power in the reactance-free, sine wave alternating current circuit is twice the effective power. There are other instants in time when VA power is zero; at still other moments, VA power is somewhere between zero and twice the effective power level. The constantly changing power is termed as instantaneous power.

In some situations, like with the voice modulated radio signal or a fast scan television signal, instantaneous power varies in the extremely complicated fashion. Perhaps you have seen modulation envelope of such a signal displayed on the oscilloscope.

Figure--   Peak versus effective power for a sine wave.