The rating of the transformer is expressed in kVA. The copper losses of a transformer are variable losses and depends upon the current rating of the transformer where the hysteresis and eddy current (the iron) losses are depending upon the voltage. In other words the total losses of a transformer are depending upon the voltage and current ratings of the transformer. So the capacity of the transformer is rated in kVA to indicate the maximum current rating on that particular voltage irrespective to the load power factor. In A.C. the power is the product of voltage current and power factor. The power factor depends upon the load. So the VIcosΦ will not give any constant value and will depend upon the power factor. That is the reason the transformer are rated in k VA and not in kW.