This paper presents a literature study of the degradation of flat plate solar collectors used in Domestic Hot Water (DHW) systems. The scope of the paper is limited to the selective absorber coating of the collector and is focused on the research made in the IEA Task X study. The objective is to investigate the potentials of computer simulations of performance reduction of a solar-assisted heat pump system, due to absorber coating degradation. The Task X study used an approach of accelerated testing to access the extent of absorber coating degradation. Mathematical models describing the coating degradation were used to transform the accelerated results into in-service conditions. A mathematical model of the microclimate in a flat plate collector was also developed. Results showed that the accelerated test procedure is in fairly good agreement, although it contains a number of simplifications, when compared with specimens from in-service collectors that have been working for 3 to 15 years. By using in-service microclimate data and accelerated test results, calculations into approximated real time degradation can be made. The microclimate model that was intended for simulations gave deviating results when compared with measurements, showing that further development is needed. The methodologies from the Task X study could also be useful when assessing the effects of absorber coating degradation on other types of collector systems, e.g. a solar-assisted heat pump system. Such a system is under evaluation in the ongoing EU project named: Endothermic Technology for Energy Efficient Housing in the EU (ENDOHOUSING). The project uses solar-assisted heat pumps to provide the thermal energy to meet space heating, cooling and hot water requirements for domestic houses in different regions of the EU throughout the year. Six demonstration houses (endohouses) will be established and equipped accordingly across the EU and evaluated during the project. If these systems are to be commercially installed and used they must be economically feasible. This places emphasis on the cost, durability and performance of the system. A solar collector is exposed to various strains that will contribute to a degradation of the collector materials, which will decrease its energy performance. These changes will in turn be transposed throughout the system lowering its degree of efficiency that directly relates to the economical aspect of the system. As a result of these changes a performance over time assessment is needed for this type of system, which in turn is in line with the European Construction Products Directive (CPD).