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.
Also read here about :
How to Calculate/Find the Rating of Transformer (Single Phase and Three Phase)?
TRANSFORMER NAMEPLATE (GENERAL REQUIREMENTS).