AC power terminology — is apparent power expressed in watts (W)?

Difficulty: Easy

Correct Answer: False

Explanation:


Introduction / Context:
Correct AC power units are critical for specifying transformers, UPS systems, and inverters. Apparent power, real power, and reactive power use different units and convey different physical meanings.


Given Data / Assumptions:

  • Sinusoidal steady-state conditions.
  • Three standard power quantities: real (P), reactive (Q), and apparent (S).


Concept / Approach:
Real power P is measured in watts (W). Reactive power Q is measured in volt-ampere reactive (var). Apparent power S is measured in volt-amperes (VA). Apparent power reflects the product of RMS voltage and RMS current irrespective of phase angle.


Step-by-Step Solution:

Define S = V_rms * I_rms (unit VA).Define P = V_rms * I_rms * cos(phi) (unit W).Define Q = V_rms * I_rms * sin(phi) (unit var).Power triangle relation: S^2 = P^2 + Q^2.


Verification / Alternative check:
Equipment nameplates often list kVA for transformers (apparent power rating) and kW for loads (real power). This practical distinction validates the unit separation.


Why Other Options Are Wrong:

  • Choosing “True” conflates apparent power with real power; only P uses watts.


Common Pitfalls:
Using kW when sizing generators for loads with poor power factor; the correct initial sizing metric is kVA, then ensure adequate kW capacity given expected PF.


Final Answer:
False

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