Differentiate between real, reactive, and apparent power in AC circuits.

Real power (P, watts) is the actual power consumed doing useful work, calculated as VIcos(phi). Reactive power (Q, VARs) represents energy stored and returned by inductors and capacitors, calculated as VIsin(phi). Apparent power (S, VA) is the vector sum: S = sqrt(P^2 + Q^2) = VI. The power triangle relates these quantities, with the angle phi representing power factor angle.