This paper presents a numeric model for proton exchange membrane electrolysis, which describes the current-voltage dependency. Besides physical constants and parameters characterizing material properties, it also contains parameters that cannot be measured ex situ, e.g., charge transfer coefficients and exchange current densities. To determine these parameters, the model includes an automated parameter calibration procedure that is able to find the best parameter combinations. An exemplary integration of the model in MATLAB/Simulink is presented, including a validation with dynamic measurement data.