Why transformer rating in KVA?

        As we know Copper loss in the transformer is depends on the current and core loss in the transformer is depends on the voltage.Hence total transformer loss is depends on Volt Ampere  (VA) and not on the phase angle between the current and voltage. ie, It is not depend on the load power factor.This why transformer rating in KVA and not in KWh.
       There is also another saying,If we consider an electric motor we can predict its output power.Beyond that particular power it doesn't work.The transformers are used in generation and distribution side so we cant predict the load on the transformer as that in the case of an electric motor.The load on the transformer may be lagging,leading or unity power factor.Hence we use KVA rating for transformer.
       The rated KVA of the primary winding and the secondary winding are equal.Hence we can easily calculated the rated current from the rated KVA and the corresponding rated voltage.