Imagine a single phase system with a sinusoidal voltage source which load is a RC circuit. As under such conditions, the elements’ behaviour is linear, the source current waveform will be sinusoidal, but due to the properties of capacitor it will be shifted in relation to source voltage. In such a system, reactive power Q can be interpreted as an amplitude of energy oscillation which alternately is collected in the capacitor and returned to the source. This is typically shown on the power quality analyser as the appearance of reactive power being a phase shift between the current and voltage waveform.
How is Reactive power measured on sinusoidal circuits?