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This video explains why transformers are rated in kVA and not in kW. The same logic discussed in the video can also be applied to generators to understand why generators are rated in kVA and not kW, and why motors are rated in kW and not kVA.

For the purposes of this video we assume the generator is an alternator. Equations given are for single phase circuits, three phase equations are slightly different.

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▶️Why are transformers and generators rated in kVA and not kW?

Because transformer and generator manufacturers do not know the load to which the generator or transformer will be connected. Loads are either resistive, inductive, capacitive, or a mixture of these. The type of load dictates the power factor. If the power factor is known, a rating can be given in kW, but, the power factor is not known because the load is not known, therefor it is not possible to give the transformer or generator a rating in kW. It is for this reason that transformers and generators are rated in kVA. The below equation shows the relevance of calculating kW when using the power factor.

VA = Volts x Amps

kVA = (Volts x Amps) / 1000

If voltage and current in phase then:

VA=Watts

If not in phase then:

kW = VA x p.f.

If thought about in a different way. The kVA rating is apparent power and kW is real power (actual power). Apparent power is the power available assuming no load losses, but there are always losses, so this value is only of use theoretically. Once we apply the load, we can calculate the power factor. Power factor is the ratio of theoretical work output compared to real work output. We can calculate real work by multiplying the apparent power by the power factor e.g. 100 kVA x 0.8 = 80 kW

Without knowing the type of load, the power factor cannot be known and thus no value for kW can be given for either the transformer or generator.

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