Why is Transformer Rated Capacity Expressed in Apparent Power, While Generator Rated Capacity is Expressed in Active Power?

Recently, an on-site maintenance technician asked about the power factor of transformers. I explained it by discussing why transformers use apparent power to represent their capacity. Below, we’ll explore this topic in two parts:

​Part 1: Relationship Between Apparent Power, Active Power, and Their Units

For those with a basic understanding, you can skip this section and proceed to Part 2.

First, let’s clarify a concept: ​W (watt) and ​VA (volt-ampere) are dimensionally the same unit, as is ​var (volt-ampere reactive).

​kW is the unit for ​active power (P).

​kVA is the unit for ​apparent power (S).

​kvar is the unit for ​reactive power (Q).

In AC systems:

The ​dot product of voltage and current represents ​active power (P), measured in ​W.

The ​cross product of voltage and current represents ​reactive power (Q), measured in ​var.

The product of the ​RMS values of voltage and current represents ​apparent power (S), measured in ​VA, expressed as ​S = U × I.

The relationship between these three is given by the equation: ​S² = P² + Q².

​Active power is ultimately converted into various forms of energy dissipation, such as mechanical or thermal energy.

​Reactive power is exchanged between inductors and capacitors and is not consumed under ideal conditions.

​Part 2: Why Are the Units for Transformers and Generators Different?

​Transformers

​Function: Transformers change voltage and current, not electrical energy. They have minimal active power losses (ignoring iron and copper losses), meaning the capacity on both sides is the same.

​Load Characteristics: Transformer loads can be resistive, inductive, or capacitive, resulting in varying power factors. This means the output power includes both active power and reactive power (used to establish magnetic or electric fields).

​Unified Measurement: Since transformers can connect to different types of loads, the power factor varies. ​Apparent power remains a constant value, unaffected by load characteristics, making it a unified measure of the transformer’s maximum transmission capacity. Therefore, using apparent power to represent transformer rated capacity more accurately reflects its load-bearing capability and design limits.

Alternatively, if transformers were labeled with active power, the load characteristics would be unknown, making such labeling impractical.

​Generators

​Function: Generators are designed to produce ​active power while also regulating reactive power. Hence, generators are primarily concerned with active power, and their rated capacity is expressed in active power units.

​Practical Use: In most applications, users are interested in how much active power a generator can provide, as it directly determines the actual load it can drive. This is similar to the case of electric motors.

​Conclusion

Transformers and generators use different power indicators to represent their rated capacity due to their distinct roles in the power system. This approach better aligns with their practical applications and performance evaluation needs.

​Transformers: Use ​apparent power (kVA) to reflect their ability to handle varying load characteristics.

​Generators: Use ​active power (kW) to indicate their capacity to drive actual loads.

In practical applications, it is also possible to convert the rated capacity of generators to apparent power or transformers to active power as needed.

Let me know if further clarification or adjustments are required!