Transformers are rated in kVA (kilovolt-amperes) instead of kW (kilowatts) for a few key reasons:
Power Factor Independence: The output of a transformer is the product of voltage and current, without considering the phase angle between them. Since the power factor (which indicates the phase difference between the voltage and current) is not constant and can vary with the load (it can be lagging, leading, or unity), the actual real power in kW can change based on the type of load connected. Rating a transformer in kVA means it can accommodate any kind of load, irrespective of the power factor, as it doesn’t factor in the phase angle.
Load Versatility: Different types of loads (resistive, inductive, or capacitive) will draw different types of current (in phase, lagging, or leading). Since kVA is a unit of apparent power that includes both active (real) and reactive power, a transformer rated in kVA can be used with any mix of loads without specifying the power factor.
Simplicity in Design: Transformer losses can be broadly classified into two categories: copper losses (which depend on the current) and iron losses (which depend on the voltage). Since these losses do not directly depend on the power factor, it is simpler to rate and design transformers based on the apparent power (voltage times current) which is a measure of the total electrical power flowing through the transformer.
Thermal Considerations: The heating of the transformer components is mainly a function of current, which in the case of a transformer is accounted for in the kVA rating since it’s the product of voltage and the maximum current it can handle, regardless of phase angle. A transformer’s size and cooling systems are designed to dissipate heat based on the total electrical energy (in kVA) it is expected to handle.
Simply put, a kVA rating tells you the maximum amount of apparent power the transformer can handle without taking into account how the energy is being used by the load. This makes it a universal rating that applies to all kinds of loads, whether they are light bulbs (which have a power factor close to 1) or induction motors (which have a lagging power factor).