Return to power (idea)
We were all¹ taught in the [electicity] module of our [Physics] [class] that:
P = VI (Power = [Volts] × [Amps])
This is [true], but with a clarification which is glossed over in all but the most advanced courses.
For AC power, I and V are [vector] quantities, not scalar. The [peak] [current] may not be drawn at the same time as the [potential difference] peaks, especially if the [load] is [reactive], rather than just [resistive].
Imagine two [sine] waves which may or may not be out of [phase] with each other. If the waves are in phase, then our P=IV equation holds. If the waves are [exactly] out of phase, then P=0. However much power we supply, no [work] will get done. All electrical equipment has a power factor between these extremes, with [motor|motors] generally being worse than solid-state equipment.
This [power factor] is very important in machines which use a large amount of [electricity]; but can generally be [ignore|ignored] on small equipment. A low power factor means that the [power station] has to supply current at times when potential is low, and can [damage] their equipment. For this reason, large [factory|factories] have to [measure] and [declare] their power factor.
Electrical equipment may be [rate|rated] in one or more ways.
W = PowerFactor × VA
If VA, but not the power factor is supplied, then we can still work out a [worst-case] heat dissipation; it will always be VA or better.
¹ Those who took Physics, at any rate.