Video narration
In a purely resistive load, voltage and current rise and fall together, so energy flows one way from the source into heat or useful work. Now add an inductor. Current lags, because the inductor stores energy in a magnetic field while the current builds, then releases that energy later in the cycle. A capacitor does the mirror image: current leads, as it stores energy in an electric field while it charges, then sends it back as it discharges. Over a full AC cycle, an ideal inductor or capacitor uses zero net energy—but energy still shuttles back and forth between source and load. That back-and-forth energy flow is reactive power, measured in VAR. Even with no net work, the reactive current still travels through the wires, causing I-squared-R heating, extra voltage drop, and forcing generators, transformers, and cables to be sized for the larger apparent power. Power factor tells how much of the current is doing real work versus just moving energy in fields.