A device, having: a signal interface, a processor unit, and a memory unit, wherein the signal interface is designed to receive a measurement signal representing the first sensor data and second sensor data, wherein the first sensor data and the second sensor data in each case represent at least one quantity detected in or at the power transformer by means of sensors, wherein the processor unit is configured to produce in each of successive time windows a measurement dataset from the first and second sensor data received in the given time window, wherein in the memory unit is stored a simulation model of the power transformer, which simulation model describes a transfer behavior for input data into output data as a function of model parameters of the simulation model, wherein the processor unit is configured to execute for each time window the following group of steps a) to c) repeatedly as a group: a) determining output data by executing the simulation model using input data, which is defined by first sensor data from the measurement dataset assigned to the given time window, and using model parameters, which, for the first execution of the simulation model for the given time window are defined by predefined reference parameters, and otherwise by the most recently determined optimization parameters, b) determining a target value, which comprises at least one squared error, weighted by a first weighting factor, which error is between the second sensor data from the measurement dataset assigned to the given time window and the previously determined output data, and c) determining optimization parameters as new model parameters for the simulation model on the basis of the at least one target value determined in the given time window, wherein the simulation model also describes a model value for a temperature inside the power transformer as a function of input data and model parameters, and wherein the processor unit is configured to determine for each time window the model value by executing the simulation model using input data, which is defined by first sensor data from the measurement dataset assigned to the given time window, and using model parameters, which for the given time window are defined by the most recently determined optimization parameters. a signal interface, a processor unit, and a memory unit, wherein the signal interface is designed to receive a measurement signal representing the first sensor data and second sensor data, wherein the first sensor data and the second sensor data in each case represent at least one quantity detected in or at the power transformer by means of sensors, wherein the processor unit is configured to produce in each of successive time windows a measurement dataset from the first and second sensor data received in the given time window, wherein in the memory unit is stored a simulation model of the power transformer, which simulation model describes a transfer behavior for input data into output data as a function of model parameters of the simulation model, wherein the processor unit is configured to execute for each time window the following group of steps a) to c) repeatedly as a group: a) determining output data by executing the simulation model using input data, which is defined by first sensor data from the measurement dataset assigned to the given time window, and using model parameters, which, for the first execution of the simulation model for the given time window are defined by predefined reference parameters, and otherwise by the most recently determined optimization parameters, b) determining a target value, which comprises at least one squared error, weighted by a first weighting factor, which error is between the second sensor data from the measurement dataset assigned to the given time window and the previously determined output data, and c) determining optimization parameters as new model parameters for the simulation model on the basis of the at least one target value determined in the given time window, a) determining output data by executing the simulation model using input data, which is defined by first sensor data from the measurement dataset assigned to the given time window, and using model parameters, which, for the first execution of the simulation model for the given time window are defined by predefined reference parameters, and otherwise by the most recently determined optimization parameters, b) determining a target value, which comprises at least one squared error, weighted by a first weighting factor, which error is between the second sensor data from the measurement dataset assigned to the given time window and the previously determined output data, and c) determining optimization parameters as new model parameters for the simulation model on the basis of the at least one target value determined in the given time window, wherein the simulation model also describes a model value for a temperature inside the power transformer as a function of input data and model parameters, and wherein the processor unit is configured to determine for each time window the model value by executing the simulation model using input data, which is defined by first sensor data from the measurement dataset assigned to the given time window, and using model parameters, which for the given time window are defined by the most recently determined optimization parameters.