Why do we use kVA instead of kW to rate transformers?

The use of kVA (kilovolt-amperes) instead of kW (kilowatts) to rate transformers stems from the fundamental distinction between real power (kW) and apparent power (kVA) in electrical systems. Transformers transfer electrical energy between circuits via electromagnetic induction, and their kVA rating accounts for both real and reactive power.

Real Power (kW): This is the actual power that performs useful work—such as producing mechanical energy, heat, or light—and reflects the transformer's energy delivery capability.

Reactive Power (kVAR): While not performing useful work, reactive power is essential for maintaining voltage levels and ensuring system stability. Transformers inherently require magnetizing current, which introduces reactive power.

Apparent Power (kVA) is the vector sum of real power (kW) and reactive power (kVAR). Rating transformers in kVA provides a comprehensive measure of their total power-handling capacity. This is particularly important in systems with inductive or capacitive loads—such as motors—that require both real and reactive power.

In summary, specifying transformer ratings in kVA—rather than kW—recognizes the combined effect of real and reactive power. It offers a more accurate representation of the transformer’s ability to handle total power flow, including the reactive component critical to system stability and efficiency.