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Transformers are rated primarily based on apparent power, which is the product of the voltage and current in a system. This rating is expressed in volt-amperes (VA) and provides a measure of the total power that a transformer can handle, irrespective of the phase angle between the voltage and current.

In an electrical system, apparent power encompasses both real power, which represents the actual work done or the energy consumed, and reactive power, which is associated with the energy stored and released by inductive and capacitive elements. Since transformers operate with alternating current (AC), they need to deal with both real and reactive power, but their primary rating focuses on the total or apparent power because it reflects the maximum load they can handle without overheating.

Using apparent power as a rating allows engineers to accurately assess the transformer's capacity to transfer energy effectively without considering the power factor, which can vary with different loads and conditions. Thus, the use of apparent power in transformer ratings ensures that they can manage diverse loads efficiently while maintaining reliability and safety in electrical systems.