Why Transformer Rating In kVA, Not in KW?

Why Transformer Rating In kVA, Not in KW?

In Simple words, 

There are two type of losses in a transformer:

1. Copper Losses

2. Iron Losses or Core Losses or  Insulation Losses

Copper 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.