This master thesis proposes a model of an alkaline electrolysis plant. The aim is to determine the performance of a complete hydrogen production system at different working conditions. An explanation and characterization of the fundamental aspects of water electrolysis is carried out and a modeling approach is proposed for the interpretation of the system. Semi-empirical equations are used for describing the polarization behaviour, the Faraday efficiency and the gas purity variation with the current. The required parameters are calculated with a retrofitting procedure on Matlab, on the basis of experimental data. The electrolyzer stack block is defined in Aspen Custom Modeler and exported to Aspen Plus, where it is implemented in the complete plant.