Transformers are usually rated in units called __________.

• A. Volt amperes
• B. Watts
• C. Joules
• D. Ohms

Answer: (A)

The key idea is that transformers are sized by apparent power, measured in volt-amperes (VA). This reflects the actual energy the windings must handle as voltage and current flow, regardless of how much of that power is useful (real) energy at any given moment. The heating and thermal limits depend on current level and the resulting copper losses (I^2R), which are tied to RMS current and voltage, not to how effectively the power is used.

Because the load’s power factor can vary, the real power (watts) delivered can swing while the transformer’s heating—and thus its safe rating—remains governed by the VA. That’s why transformers are specified in VA or kVA rather than watts; it provides a consistent, PF-independent measure of capacity.

Joules measure energy, not a continuous rating, and Ohms measure resistance, not capacity. Watts can be misleading for sizing because they don’t account for the phase relationship between voltage and current.