EASA / CAR Part-66 · Module 3 · 3.14 R, C and L Circuits · Cat B1/B2
When current lags or leads voltage (a reactive load), not all the supplied power does useful work. The phase angle φ sets the power factor cos φ, and splits apparent power into real and reactive parts.
Real power P = V·I·cos φ (watts, does work). Reactive power Q = V·I·sin φ (VAR, sloshes back and forth). Apparent power S = V·I (VA, what the supply must deliver). A power factor near 1 is efficient; a poor factor means extra current for the same useful power.