Abstract: Method and device for determining an insulation resistance RISO of a photovoltaic system (1) with respect to earth potential (EP), comprising a DC-DC converter unit (4) which converts a first DC voltage generated by the photovoltaic generator (2) into a second DC voltage which is applied to an intermediate circuit (5) that temporarily stores the electrical energy, supplied by the photovoltaic generator (2), for a DC-AC converter unit (6) connected thereto, wherein the intermediate circuit (5) contains a reference node (BK) having an adjustable reference potential (BP), and comprising a measuring unit (7) which picks up the measuring voltages between the reference potential (BP) and the earth potential (EP) at different reference potentials (BP), and an evaluation unit (8) which evaluates the measuring voltages that have been picked up in order to determine the insulation resistance of the photovoltaic generator (2) of the photovoltaic system (1).