Block-oriented modelling is a powerful tool in system design. The models developed this way can be used to study various systems, such as solar thermal systems. In the work presented, main goal is to compare different mathematical models of solar thermal devices. The difference in accuracy of such models in respect to the possibilities of a hardware-in-the-loop simulation will be discussed. Two models of solar thermal collectors will be compared and used in a system containing two different heat storages in order to study the behaviour of these models. The tests were carried out in a Simulink based block-oriented environment called SimSolar.