What does S stand for in power calculations?

• A. Apparent power
• B. Real power
• C. Reactive power
• D. Voltage

Answer: (A)

In AC power calculations, S stands for apparent power. It is the magnitude of the complex power P + jQ, representing the total power that must be supplied, regardless of phase. It equals V_rms times I_rms, combining both the work-producing part and the reactive energy exchange. The relationship S = sqrt(P^2 + Q^2) shows why apparent power can be larger than real power when reactive power is present. The power factor cos(phi) = P/S links real power to apparent power. If there were no reactive component, S would equal P. So S is not real power or reactive power, nor is it voltage—it is the total, apparent power.