Why transformer is Rated in KVA rather than 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 losses ( I²R)depends on Current which passing through transformer winding while Iron Losses or Core Losses or  Insulation Losses depends on Voltage. Hence total transformer Loss depends on volt-amper ( VA )  not on phase angle between voltage and current means that it is idependent of load power factorThat’s why the Transformer Rating is  in kVA,Not in kW.

Due to the fact that Core losses/Iron losses/Insulation losses are based on input voltage 

Copper losses ( I²R) depends on Current which passing through transformer winding while Iron Losses or Core Losses or Insulation Losses depends on Voltage. So the Cu Losses depend on the rating current of the load so the load type will determine the powerfactor P.F , Thats why the rating of Transformer in kVA,Not in kW.

transformer is used for transfer power from one level lo another level or step up or step down fromone circuit to an other circuit not consuming power so transformer always raed in kva

kVA is the unit for apparent power. Apparent power consists of active and reactive power. Active power is the share of the apparent power which transmits energy from the source (generator) to the user. Reactive power is the share of the apparent power which represents a useless oscillation of energy from the source to the user and back again. It occurs when on account of some »inertia« in the system there is a phase shift between voltage and current. This means that the current does not change polarity synchronous with the voltage. But the heat generated in a winding as well as the eddy current losses generated in a transformer core depend on the current only, regardless of whether it aligns with the voltage or not. Therefore the heat is always proportional to the square of the current amplitude, irrespective of the phase angle (the shift between voltage and current). So a transformer has to be rated (and selected) by apparent power.

In case of transformers and generators, the rating is in KVA. Because in this case the total power that can be distributed by the equipment has importance which is KVA.

While loads are rated in KW. KW is the active power which is consumed effectively. For instance, in case of motor and other loads the user should know what active power it can deliver.

Copper loss of transformer depends on current and Iron loss on voltage. Hence the total transformer loss depends on volt-ampere(VA) and not on phase angle between voltage and current i.e. it is independent of load power factor. That is why rating of transformer is in kVA and not in KW