A test facility for solar domestic hot water systems, SDHW systems was established at the Technical University of Denmark in 1992. During the period 1992-2012 21 marketed SDHW systems, 16 systems from Danish manufacturers and 5 systems from manufacturers from abroad, have been tested in the test facility under the same realistic test conditions. The systems had different designs and sizes. Each system was tested during a long test period consisting of both a summer and winter period. Detailed simulation models for each system were developed. The simulation models were modified and the input to the models were fitted in such a way, that the calculated thermal performance is in good agreement with the measured thermal performance, both for a typical winter period and for a typical summer period. In this way it is possible to use the simulation models to calculate the yearly thermal performance of the tested systems with weather data from the Danish Test Reference Year and with the same hot water consumption. The tests showed that the designs of the heat storage and that the system concepts are of vital importance for the thermal performances of the systems and that neither the solar collector efficiency nor the solar collector area is influencing the thermal performance as much as the heat storage design and the system concept. The tests also showed that all the tested systems can be improved with relative simple design changes. Based on the tests it is concluded that high thermal performances of SDHW systems are achieved by reducing the heat loss from the upper part of the heat stores to a minimum by having no pipes connected to the upper part of the tank, reducing the auxiliary volume at the top of the heat stores as much as possible, of course with consideration of the required hot water comfort, avoiding simple errors, using the low flow principle and heat stores with a high degree of thermal stratification and by using components with good thermal characteristics.