A dynamic model for a hybrid Photovoltaic Thermal Collector-Solar Air Heater (PVT-SAH) with longitudinal fins was developed to enable assessment of the potential of the system to provide high temperature outlet air (60–90 °C) under dynamic boundary conditions. The model description includes the method for discretising the system into a number of control volumes, the energy balance equations for each control volume and the implementation of the numerical solution. Model validation has been successfully undertaken by using empirical verification of model predictions with an experimental facility and by comparing the model outputs with the reference data from the literature. The dynamic PVT-SAH model was then used under variable boundary conditions and its performance was compared with an equivalent steady state model. Significant Time Constants (TC) were observed and it was found that the steady state model could overestimate the thermal energy gains of PVT-SAH by 35% when compared with the predictions of the dynamic model. Additional simulations were run under fixed boundary conditions to shown the effect of fins on the performance of the PVT-SAH system. Finally, to demonstrate the benefits of using such a dynamic PVT-SAH model, a case study was used and the effect of length ratio of PVT to SAH was investigated by using a range of performance criteria.