Load ratings for transformers are usually given as VA or kVA, these are obtained by multiplying the load current by the supply voltage: this gives the volt-ampere (VA) rating, where 1000 VA = 1 kVA. This is often loosely referred to as the "power" rating of the transformer. In actual practice the power which the transformer can deliver into a given load also depends on a quantity known as the power factor (p.f.), which is defined as the ratio of real power to apparent power, i.e. the absorbed power divided by the volt-ampere (VA) product. For a purely resisitive load the p.f. is unity and the transformer will be able to supply a load power equal to its VA rating. In practice the majority of loads are not purely resistive and have a small reactive (inductive or capacitive) component which should be taken into account. When a p.f. value is not known it is common practice to assume a value of 0.8.

Filament lamps and coiled heater elements may be assumed to have a power factor of approximately 0.95, which is often ignored when specifying a trasnsformer. Discharge lamps (which includes fluorescent lamps) present a lower power factor, which in industrial and commercial situations is normally adjusted by the connection of a p.f. correction capacitor to a value of 0.95. Uncorrected power factors for discharge lamps can be as low as 0.5, so it pays to correct.