The scope of this work is to ascertain if the inclusion of Phase Change Materials (PCM) in the thermal energy storage of a Solar Domestic Hot Water System (SDHW) could be beneficial for increasing the energy savings of the system or in reducing the space occupied by the thermal energy storage. A simple SDHW plant is studied which features a plane solar collector, a boiler and a PCM enhanced tank. The PCM improved storage tank has been optimized using mono and multi-objective genetic algorithms. The optimization has been carried out with the modeFRONTIER optimization tool, while the system plant has been analyzed by means of a modified version of the building energy simulation code ESP-r. In parallel with the optimization a sensitivity analysis has been carried on in order to find out the relation between the design parameters of the tank (geometry, phase change temperature of the PCM, and insulation) and the performance of the system. Thanks to the multi-objective optimization of the system different solutions with different rankings of the optimized variables have been presented. The main result is that for this application the PCM has not the major impact on the results, while other parameters play a more significant role. (C) 2014 Elsevier Ltd. All rights reserved.