Therefore, the instantaneous power is given by a constant value VI plus the ac quantity oscillating with twice the angular frequency of the power supply; thus, the active power is simply the product VI.
In this case, all the above considerations referring to active power for dc circuits are still correct, but voltages and currents must be replaced by the corresponding rms values.
The case of purely reactive loads is the opposite; the voltage drop across the load and current flowing through it are out of phase by 90°.
Instantaneous power p(t) is given by: