Device for determining insulation resistances of power networks (106, 108) with respect to an electrical earth in a motor vehicle, wherein the power networks (106, 108) are coupled to one another by way of at least one power converter, wherein the at least one power converter comprises an inverter and/or a DC-DC converter, the device comprising: a control device which is designed, in at least two measuring intervals, to control a respective operating state of the at least one power converter of the motor vehicle, which is conductively connected to a traction energy store; a measuring device with a measuring terminal, which is conductively connected or connectable to at least one DC voltage pole of the electric traction energy store of the motor vehicle, and with an earth terminal, which is conductively connected or connectable to a reference potential of the motor vehicle, wherein the measuring device is designed, in the at least two measuring intervals, to respectively measure a conductance between the measuring terminal and the ground terminal; and a calculation device, which is designed to determine the insulation resistances as a function of the at least two measured conductances and the at least two operating states controlled, wherein the operating state comprises a duty cycle of at least one switching element of the power converter, wherein the duty factor controlled is less than 100% and greater than 10%, and wherein the function incorporates parameters for the weighting of a dependence of the insulation resistance on the operating state controlled, and wherein the calculation device incorporates a memory, in which motor vehicle-specific parameter values are saved, characterized in that the control device defines at least one measuring interval of the at least two measuring intervals in such a way that it precedes driving readiness being established and/or follows driving readiness of the motor vehicle being withdrawn. a control device which is designed, in at least two measuring intervals, to control a respective operating state of the at least one power converter of the motor vehicle, which is conductively connected to a traction energy store; a measuring device with a measuring terminal, which is conductively connected or connectable to at least one DC voltage pole of the electric traction energy store of the motor vehicle, and with an earth terminal, which is conductively connected or connectable to a reference potential of the motor vehicle, wherein the measuring device is designed, in the at least two measuring intervals, to respectively measure a conductance between the measuring terminal and the ground terminal; and a calculation device, which is designed to determine the insulation resistances as a function of the at least two measured conductances and the at least two operating states controlled, wherein the function incorporates parameters for the weighting of a dependence of the insulation resistance on the operating state controlled, and wherein the calculation device incorporates a memory, in which motor vehicle-specific parameter values are saved, characterized in that the control device defines at least one measuring interval of the at least two measuring intervals in such a way that it precedes driving readiness being established and/or follows driving readiness of the motor vehicle being withdrawn.