Method for determining a currently maximum possible power (P max) of a photovoltaic, PV, installation operated in a curtailed manner and comprising a PV generator having at least one string, wherein the at least one string comprises a series circuit of PV modules, in which at least one PV module is connected to adjacent PV modules in the string via a coupling unit, an inverter which is connected, on the output side, to an energy supply grid and is connected, on the input side, to the PV generator, a controller (7, 7.1) for controlling the inverter and the coupling unit, wherein the controller (7, 7.1), in conjunction with the coupling unit, is designed to vary a power drain of the PV module assigned to it relative to a power drain of the adjacent PV modules during operation of the PV installation, having the steps of: - determining reference values for the PV module assigned to the coupling unit, comprising a reference MPP power (P MPP,Ref) and at least one value, from a short circuit reference current (I SC,Ref) and/or an open circuit reference voltage (U 0,Ref), that is assigned to the reference MPP power (P MPP,Ref), - operating the PV installation, in response to a curtailment signal, in a curtailed state with a lower power than the currently maximum possible power (P max) of the PV installation, characterized in that - for the PV module assigned to the coupling unit in the curtailed state of the PV installation, - a second current value I SC,2) indicating a short circuit operating mode is determined, and/or - a second voltage value (U 0,2) indicating an open circuit operating mode is determined, and - wherein the currently maximum possible power (P max) in the curtailed state of the PV installation is determined from - the second current value (I SC,2), the short circuit reference current (I SC,Ref) and the reference MPP power (P MPP,Ref), or - the second voltage value (U 0,2), the open circuit reference voltage (U 0,Ref) and the reference MPP power (P MPP,Ref), or - a combination of the second current value (I SC,2), the short circuit reference current (I SC,Ref), the second voltage value (U 0,2), the open circuit reference voltage (U 0,Ref) and the reference MPP power (P MPP,Ref). - determining reference values for the PV module assigned to the coupling unit, comprising a reference MPP power (P MPP,Ref) and at least one value, from a short circuit reference current (I SC,Ref) and/or an open circuit reference voltage (U 0,Ref), that is assigned to the reference MPP power (P MPP,Ref), - operating the PV installation, in response to a curtailment signal, in a curtailed state with a lower power than the currently maximum possible power (P max) of the PV installation, characterized in that - for the PV module assigned to the coupling unit in the curtailed state of the PV installation, - a second current value I SC,2) indicating a short circuit operating mode is determined, and/or - a second voltage value (U 0,2) indicating an open circuit operating mode is determined, and - a second current value I SC,2) indicating a short circuit operating mode is determined, and/or - a second voltage value (U 0,2) indicating an open circuit operating mode is determined, and - wherein the currently maximum possible power (P max) in the curtailed state of the PV installation is determined from - the second current value (I SC,2), the short circuit reference current (I SC,Ref) and the reference MPP power (P MPP,Ref), or - the second voltage value (U 0,2), the open circuit reference voltage (U 0,Ref) and the reference MPP power (P MPP,Ref), or - a combination of the second current value (I SC,2), the short circuit reference current (I SC,Ref), the second voltage value (U 0,2), the open circuit reference voltage (U 0,Ref) and the reference MPP power (P MPP,Ref). - the second current value (I SC,2), the short circuit reference current (I SC,Ref) and the reference MPP power (P MPP,Ref), or - the second voltage value (U 0,2), the open circuit reference voltage (U 0,Ref) and the reference MPP power (P MPP,Ref), or - a combination of the second current value (I SC,2), the short circuit reference current (I SC,Ref), the second voltage value (U 0,2), the open circuit reference voltage (U 0,Ref) and the reference MPP power (P MPP,Ref).