Why is a transformer rated in kVA and not in kW?

Think about which losses depend on voltage and which depend on current.

Model answer.

A transformer's rating is set by how hot it gets, and its heating comes from its losses. There are two main kinds:

1. Core (iron) loss, made up of hysteresis and eddy-current loss. It depends on the flux in the core, which depends on the applied voltage and frequency.
2. Copper (I²R) loss in the windings, which depends on the current.

Neither loss depends on the power factor of the load. A transformer delivering 100 A at 0.6 power factor has exactly the same copper loss as one delivering 100 A at unity power factor, even though the real power (kW) is quite different. Because the temperature rise is fixed by voltage and current, the manufacturer rates the transformer by the product of the two: volt-amperes (VA, kVA or MVA).

The manufacturer also does not know what load will be connected. The power factor is decided by the customer's load, not by the transformer, so a kW rating would be meaningless without a stated power factor.

Contrast with a motor. A motor is rated in kW (or hp) because its nameplate states the mechanical output at the shaft, and that is what the buyer needs to know. Alternators, like transformers, are rated in kVA for the same reasons as above.

Quick example. A 100 kVA transformer can supply 100 kW at unity power factor but only 80 kW at 0.8 power factor. In both cases it carries full-load current and runs at full-load temperature.

Likely follow-up. "Then why is a motor's input sometimes written in kVA?" The kVA drawn from the supply matters for sizing the cable and switchgear, while the kW rating describes what the motor delivers.