An apparatus for determining insulation resistance at a PV generator includes a first unit configured to shift a generator potential at an output terminal of the PV generator, and a second unit. The second unit is configured to determine the insulation resistance by: connecting a measurement voltage to the output terminal of the PV generator, measuring a first current value and a first voltage value at the output terminal of the PV generator before the measurement voltage is connected, measuring a second current value and a second voltage value at the output terminal of the PV generator after the measurement voltage is connected, and determining the insulation resistance of the PV generator based on the measured first and second current values and the measured first and second voltage values. The first and second units are connected in series.