An apparatus for transfer of electrical energy between a primary side and a secondary side. The apparatus includes: at least one voltage input on the primary side; at least one transformer, wherein the transformer has on the primary side at least a first winding area and a second winding area; and at least one voltage monitoring unit, wherein the voltage monitoring unit is embodied in such a manner, that it connects the voltage input with the first winding area or the second winding area as a function of a primary voltage present at the voltage input.