What are the conventional units of apparent power S?

• A. Watts (W)
• B. Volt-Amps (VA)
• C. Joules (J)
• D. Amperes (A)

Answer: (B)

The quantity S uses the product of RMS voltage and RMS current, so its unit comes directly from V × I. That makes the conventional unit the volt-amperes (VA). In AC systems the voltage and current can be out of phase, so S represents the apparent power magnitude, not the real energy transfer. The real power P, which is the actual energy-per-time delivered, uses watts (W), and the reactive power Q uses volt-ampere reactive (VAR). They’re related by P = S cos φ and Q = S sin φ, where φ is the phase angle between voltage and current. When the load is purely resistive (φ = 0), S and P are equal in magnitude, so watts and volt-amperes numerically match, but in general the units remain VA for apparent power. In practice, large systems often use kVA as a convenient unit.