A drain-back solar domestic hot water (DHW) system from SolarNor AS, Norway was built and tested in the laboratory’s test facility. The thermal performance of the system has been measured for the period from May to November 1997. A detailed simulation model have been developed for predicting the annually performance of the system. The simulation model was modified by means of comparison of the simulation results with the measurements. By using this simulation model the yearly thermal performance of the system have been investigated with the Danish Test Reference Year as the input weather data.