The unit of transformer rating is typically expressed in kilovolt-amperes (kVA) or megavolt-amperes (MVA), which represent the apparent power handling capacity of the transformer. Apparent power is a combination of real power (measured in watts) and reactive power (measured in volt-amperes reactive or VAR). The transformer's rating indicates the maximum amount of power it can handle without exceeding its thermal and electrical limits.
The transformer's rating is usually specified on its nameplate and indicates the maximum power that can flow through the transformer while maintaining acceptable temperature rise and voltage regulation. It's important to note that the transformer's actual power output (real power) will depend on the load connected to it and the power factor of that load.
In some cases, transformers might also be rated in volts-amperes (VA), especially for smaller applications, but kVA or MVA are more commonly used in larger power distribution systems and industrial installations.