If the current lags the voltage by 90° as shown in a pure inductor—
the instantaneous power shown by the heavy curve is positive in the first ½
cycle and negative in the second ½ cycle.
The power is first positive (from source to inductor) and then negative (from inductor to source). Therefore, the average power in the inductor over one full cycle is always zero, although wires are occupied with
current all the times, leaving less room for the real power to flow from the source to the load. Similarly, the average power is also zero over one full cycle in a pure capacitor, which draws current leading the voltage by 90°.
On the other hand, if the current and voltage are in phase as in a pure resistor he power is always positive even when both the voltage and the current are negative in a ½ cycle.
This results in the average power having the same positive value in both
½ cycles, and also over one full cycle.