Why Transformer Rating is in kVA, not in KW?

There are two type of losses in a transformer;1. Copper Losses2. 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. Thats why the rating of Transformer in kVA,Not in kW. 

Because the output power of transformer are Active and reactive power

KVA=KW+KVAR

Transformer rating is in KVA because transformer deals only with current and voltage, but the input power is equal to the output power.

Electrical Source is Usually rated in KVA. (Transformer is a Source)

Electrical Loads are usually rated in KW / Hp. (Motor is a Load)

Power Factor  when remained controlled (Around Unity). Power in KVA to be utilized by the Loads remains equal to rated Power KW / HP rating of Loads.

When Power Factor Decreases , KVA demand increases. As Voltages to remain same, Hence unwanted Current increases in the system & which may lead to unwanted Tripping or Burnouts in the systems.

So all Electrical Engineers & Technicians beware of Power factor of your Supervised Electrical Systems.