Why Cant Transformers and AC generators be rated in watts (W)or kilo watts (kW) or mega watts (MW). And why they should be rated in volt ampere (VA) or kilo volt ampere (kVA) or mega volt ampere (MVA) ? The main reason is, the capacity of the transformer is limited by two factors 1. the current that the transformer winding can carry and 2. the voltage it is designed to work without satuation. In general, the voltage in the power system is kept constant wihtin the specified area. Hence, the tranfromer voltage and its ratio is fixed. Therefore, the rating of the transformer is proportional to the current it can deliver to the load. And the load current is always variable and depdns on the amonut of load and also type of load. In genral, loads in the power system is a mix of resistive and reactive loads. It is a known fact that active power is related to resitive loads while reactive power is relative to reactive loads. Again reactive loads are two types, Inductive and Capacitive. Therefore, the resitive loads consumes active power and inductive loads consumes reactive power and capacitive loads produce reactive power. The rective loads do not consume active power. Even through reactive power do not consume active power, they still results in setady current flow between the source and the load. Therefore, both resitive and reactive loads invloves current flow between source and the loads. Therefore, the AC generators and transformers must supply this current ireespective of the type of the load there by loading their widnings. The load can be pure resistive, pure indtúctive or pure capacitive, or cominations of these, there will be always current flow between source and load, Hence, the transformers and AC generators are always rated in volt-amperes or VA and kilo volta amperes or Mega volta amperes.