Reactive power is the component of instantaneous power P(t)=v(t)*i(t) that have average value zero, this means that v(t) and i(t) have harmonics that aren't in phase (due to presence of reactive/storage elements at circuit), may be, pi/3, pi/4, -pi/6, pi,... All out of phase.
In RC circuit excited by a sinusoid source, in resistance v(t) and i(t) are in phase -No reactive power. but in capacitance v(t) and i(t) are out of phase by -pi/2, in time domain this mean that when v(t)=max i(t)=0 (remember V,I phasor with lag pi/2 between them), or that the maximum phase angle that the capacitance stores/devolve energy to line is pi/2. Then if mean value of energy stored in cap is zero, the number of storage times=number devolve (v(t) and i(t) different signals one >0 other <0) =2 in a cycle (excitation sinusoid source).
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