Why are transformers rated in VA instead of watts?

Transformers are rated in volt-amperes (VA) rather than watts because the power factor can vary in AC systems. Volt-amperes take into account both the voltage and current without considering the phase difference between them, which is a key factor in AC circuits. The power factor is a measure of how effectively electrical power is being converted into useful work output, and it ranges from 0 to 1.

If a transformer were rated in watts, it would imply a specific power factor (usually one), which does not account for the reality that different loads can have different power factors. By using VA, the rating remains universally applicable regardless of the load's power factor, offering a clearer understanding of the transformer's capability to handle both real power (watts) and reactive power (which does not perform any useful work). Thus, the VA rating provides a more accurate representation of the transformer's capacity in practical applications across different connected devices.