Not only Transformers, even Alternators are rated in VA, kVA or MVA etc. The rating of a Transformer depends on the winding or oil temperature reaching the specified limiting value which is decided by the type of insulation minus a safety margin, something like 170 – 20 =150 °C.
Just to clarify consider a single phase 110 V /220 V Transformer.
Let its Req = Equivalent Resistance be 3 ohm (referred to 220 V side).
Let it be rated 2.2 kVA. The rated current on 110 V side is 20 A and on 220 V side is 10 A.
First put a load on the 220 V side drawing 10 A at pf = 1.0.
The total loss will be = 3(102) = 300 W + core loss of, say 30 W.
The output will be = 220 (10) (1.0) = 2200 W = 2.2 kW.
The kVA = VI/1000 = 220(10)/1000= 2.2 kVA
Next put a purely inductive or capacitive load drawing the same 10 A but at a pf =0.
Again the total losses will be = 3(102) = 300 W+ core loss of, say 30 W, same as before.
But the output will be =220 (10) (0) = 0 kW.
The kVA =VI/1000=220(10)/1000= 2.2 kVA
So it can be seen that with the same kVA but different kW the losses are same in both cases and the temperature rise will be identical in both cases and therefore when we design a Transformer the rating of Transformer must be given in KVA or VA or MVA.