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.