The reason,why transformer rated in KVA.

• In transformer there are two 2 losses.
• one is core losses and other copper losses.
• core losses are depends on the input voltage.
• Copper loss is the one which depends on current affected by the power factor.
• So total losses depends on the volt-ampere(VA) and not on phase angle between voltage and current there fore it is independent of load power factor
• This is the reason transformers are rated in KVA and not in KW.

Single phase KVA =Amps*volts/1000. 

Single phase KW =Amps*volts * pf/1000.

3phase KVA=Amps*volts *1.73/1000. 

3 phase KW = Amps * volts * 1.73 * pf/ 1000.