A power supply, a controller, and a power element, wherein a DC compensation voltage is present at a first output of the power element to which a load having a variable current draw can be connected. A second output of the power element is thereby fed through a current measurement device, wherein a rechargeable battery is connected to the second output. A charging current or discharge current of the rechargeable battery measured by the current measuring device is set by controlling the DC compensating voltage. The charging current or discharge current of the rechargeable battery can thereby be determined even without a dedicated UPS assembly.