I'll add that the difference between watts and VA is caused by the reactive portion of a circuit, i.e. the capacitive and/or inductive portions.
Power is only consumed by the resistive part of the load; current flowing in the reactive part does not result in power being dissipated.
However the transformer needs to be sized to allow for the current flowing in the reactive part of the circuit, thus the VA rating instead of watts.
This is why it is considered a good idea to keep the power factor as close to unity as possible, so as to minimize the "wattless current" which would otherwise represent a substantial percentage of the total.
