The use of solar collectors has grown in Sweden during the last 20 years. At the present the fastest growing collector applications are Domestic Hot Water (DHW) systems. The life performance of these systems is important due to their intended long use. This applies emphasis on the system durability and service life, which is in consent with the European Construction Products Directive (CPD). It states that the essential requirements on constructed work should be met during the intended working life of the building; this results in essence in a performance requirement on all building products. The life performance of building materials and products has to be assessed and declared. This paper presents a literature study of the degradation of flat plate solar collectors used in 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 programme. The objective is to investigate the potentials of computer simulations of collector system performance reduction, 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 of the coating degradation were used to transform the accelerated test results to in-service conditions. A model of the microclimate in a flat plate solar collector was also developed. Results from the study shows that the accelerated testing 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 of approximated real time degradation can be made. The developed microclimate model, which was intended for computer simulations, gave deviating result 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, if the system performance criterion, which is related to the spectral properties of the absorber coating, is declared.