Why Transformer is rated in KVA instead of KW ?

There are two type of losses in a transformer;1. Copper Losses2. Iron Losses or Core Losses or  Insulation LossesCopper losses ( I²R)depends on Current which passing through transformer winding while Iron Losses or Core Losses or  Insulation Losses depends on Voltage.That’s why the Transformer Rating may be expressed in kVA,Not in kW.

Copper loss of the transformer depends on current and iron loss on voltage, hence total transformer loss depends on volt ampere (VA) and not on single phase between voltage and current