This paper explains the simulation and sizing optimization of renewable energy systems for hydrogen production. These systems are complex and challenging to size correctly due to the nonlinearities of the components and the stochastic nature of the sources. The same system will perform completely different at two separate sites. The simulation includes renewable energy sources, storage elements and loads. Matlab® and Simulink− are the tools utilized for the simulation and optimization. The simulation and optimization of the Renewable Energy (RenEn) system, to be constructed in Potchefstroom, South Africa, is described and some of the results obtained are discussed. The sizing of components is essential to ensure that the most efficient and cost-effective system is specified.