Why transformer rating is in kVA ? Not in kW

Transformer is not made to consume power.It is made to convert (transform from one voltage level to another voltage level) electrical power.Anything that is made for the purpose of consuming power can be rated at kW no need for kVA rating.

Now,you may argue that transformers also consumes power.It is in the form of losses in the winding and the core.

These are two types of losses in a transformer:

(1) Copper losses

(2) Iron losses or core losses or Insulation losses 

Copper losses depends on current which passing through to the transformer winding,while iron losses or core losses or insulation losses are depends on voltage.However,the losses do not depend on power factor.

Hence,to determine the complete power consumption of the transformer we need both voltage and current but not the angle between them.

That's why the transformer rating is expressed in kVA,not in kW.