Molecular catalysts are emerging as cheaper and more sustainable alternatives to traditional noble metal catalysts for fuel production. Through simulations and molecular design, the properties of molecular structures may be predicted and engineered into highly active and selective catalysts towards e.g. hydrogen evolution. However, models that can be used to compare and predict the performance of molecular catalysts are scarce.  There are ongoing attempts to translate the tools used in heterogeneous catalysis - such as the Sabatier principle and the activity volcano plot - to molecular setups, but these attempts have not yet been sufficiently connected to real-life operating conditions. In our work, we have therefore combined simulations with a thorough experimental electrchemical characterization to evaluate the catalytic properties of series of tetraphenylporphyrins. We establish that the thermodynamics of the systems can be understood and predicted with computational models, but that the thermodynamics are not sufficient to compare molecular catalysts under experimental operating conditions. The quality of the catalysts is ultimately determined by reaction kinetics, for which the theoretical tools that are currently being developed are not appropriate.