Usually we have seen that transformers and generators are rated in KVA instead of KW. We Know that KVA is a product of voltage and current and KW is a product of voltage, current and power factor. For better understanding of this topic first you have to know there are three types of loads Resistive, Inductive and Capacitive. At the of designing of transformer and alternator designer don’t know the nature of load whether it is resistive, inductive or capacitive. There is a term power factor cosΦ that multiplied with KVA and make it KW. 

Simply power factor cosΦ is defined as the cosine of the angle between voltage and current. Larger the angle between voltage and current, greater current drawn by the machine which results in losses are increased. 

From the above explanation, we have seen with change in current copper losses are changed. Copper losses are directly proportional to the square of current. Second quantity is voltage, voltage causes iron losses in the machine.

We have concluded it Copper losses ( I²R)depends on Current which passing through transformer winding while Iron Losses or Core Losses depends on Voltage. So the Cu Losses depend on the rating current of the load so the load type will determine the power factor P.F ,Thats why the rating of Transformers and alternators in kVA,Not in kW.

Energy transfer device such as transformer and energy generated device such as alternator are rated in KVA. Motors are rated in KW