The rating of a transformer is always expressed in kVA (kilovolt-amperes) because this unit represents the apparent power, which accounts for both the current and voltage flowing through the transformer, independent of the power factor of the load. Transformer losses (copper and iron losses) depend on voltage and current (apparent power), not on the real power (kW), which is influenced by the power factor. Since the manufacturer does not know the load power factor in advance, rating transformers in kW would be less practical. Using kVA ensures the rating reflects the maximum current the transformer can safely carry without overheating, regardless of whether the load is resistive, inductive, or capacitive. Key points:

• Transformer losses depend on current and voltage (apparent power, kVA), not on power factor or real power (kW).
• The power factor varies with load type (inductive, capacitive, resistive), which is unknown to the manufacturer.
• kVA rating ensures transformers can operate safely under varying load conditions without damage.
• Transformers transfer apparent power, not just real power.

Thus, kVA rating provides a realistic, flexible measure of a transformer's capacity to handle electrical loads safely and efficiently.