Hi Vanraj. This is an interesting question. You see when a manufacturer builds a transformer, he has no idea what type of load it will be used to supply. It is impossible to give a kW rating to it, without knowing the power factor of the load it will supply. The only possible way to rate it, is to assume unity pf (power factor). Transformers have copper losses and iron losses. Copper loss depends on current and iron loss depends on voltage. Neither depend on power factor. A kVA rating is more useful in determining the % loss.Volt Amp. rating, is just the Kilowatt rating determined at unity power factor (V.A = Watts / power factor). All AC devices that supply power, such as transformers, UPS, generators, etc are rated in kVA. Devices that consume power, have a kilowatt rating, because we know what its power factor is.