KW is real power, its when the current and voltage being measured are in phase ( W = V X I), i.e. they both reach a maximum at the same time. If the current and voltage do not reach their maximums at the same time, the power can be considered to have an out of phase component, which is designated KVA, or indicated power. The circuits must be rated for the KVA, because the actual currents and voltages actually flow through it, just with slightly different timings.
Frank