Solar energy has the potential that it can supply buildings with domestic hot water, among other applications. A solar thermal system with these characteristics does not need a continuous supervision, but it requires a minimal preventive maintenance which should allow keeping operating conditions within certain limits, performance, safety and durability of the installation. The aim of this work is to carry out a study based on the actual performance of Fullriggaren’s solar thermal system for domestic water since, due to the limited monitoring, its efficiency is unknown. The solar system was built in order to preheat tap water and reduce the use of district heating. Water preheated and stored is finally heated till consumption’s temperature through a heat exchange with the district heating grid of Gävle, which works as an auxiliary power supply system for the solar system. The project work focuses on all the energy conversions from the solar collector system to the storage system. Firstly, it has been done a deep study of the solar collector’s performance by making a comparison between their theoretical behaviour and actual data, which was measured for 7 consecutive during the month of June. In this purpose, WINSUN program, a simulation program based on TRNSYS, has been a useful working tool. Both theoretical and experimental result show that solar collectors are working as expected, the efficiency curve appears to be close to the tested one. On the contrary, it was observed that the heat exchanger does not transfer the heat it should; therefore it is recommended to check more carefully its performance and its level of insulation. Due to the lack of data, the work continuous by finding how much of the energy collected is actually harnessed for use. The results demonstrate that the solar system works with a solar factor of 0,62 and a total efficiency of about 44%, being the global efficiency of the thermal system 55%. In order to prove the results found, it is advised to make a large study to test the system’s performance during the most adverse periods, as this study has been carried out during few summer days.