When power transfer from primary winding to secondary winding of transformer there are two losses in transformer one is called core losses this is due to the alternating flux in core and second is copper losses this is due to current flowing in the winding when the transformer is loaded.
The I2R or copper loss is depends on current (I). And core loss is depends on voltage (V). These two losses are not depends on power factor (cos Φ) of the load. So, the temperature rise and rating of transformer is decided by the two losses one copper losses which depends on current (I) and second is core losses which depends on voltage (V). So, losses of transformer depends on V and I that's why the rating of transformer is given as the product of voltage (V) and current (I). This product of voltage and current is called as VA rating.
The VA rating of both side of transformer is same. And this rating is generally expressed in kilo volt ampere rating.
kVA = V1I1/1000 = V2I2/1000