Transformer are rated in kVA instead of kW because

Reason for kVA rating of Transformer:

Reason 1:

• The losses in a transformer include copper losses and core losses or iron losses. Core losses depend on voltage whereas copper losses depend on the current which passes through transformer winding.
• The total losses depend on volt-ampere.
   

Reason 2:

• When the transformer is designed the manufacturer does not know which type of load will be connected in the future. And the power factor depends upon the load.
• Since kVA is fixed and kW depends on the power factor.
   

Hence, the rating of the transformer is mentioned in kVA but not in kW