Asked by: Thomas Hampton, by email
An induction charger is two halves of a transformer, with a coil in each section. The mains electricity in the coil of the base induces a trickle of current in a smaller coil in the toothbrush, to charge the battery.
A perfect transformer would take no power with the two sections separated. But, in reality, coils are ‘lossy’, meaning that some power is used up as heat, even when none is being drawn via the other coil.