This study investigates the usage and prevalence of flat plate solar collectors in Denmark. The preliminary examination deals with the legislation and building regulations, the pros and cons and the different solar collectors functions. The study contains a test of the flat plate collector NIBE Solar FP215P, with the aim of determining the effectiveness on a day in November. The effectiveness was found in an interval to great to be used further in the report. The initial research problem is to investigate the relative small prevalence of the flat plate solar collector and whether a solution for a predefined purpose can be made. The result of the study is therefore an example of a flat plate solar collector system designed to fit a summer cottage and the hot water needs of 6 persons. At last the study will conclude if this type of decentralized energy collectors have a basis for a widespread use. The system requirements have been limited to include a utility water system and not a more comprehensive combination system.