The aim of the project is to develop smart solar domestic hot water (SDHW) systems. A smart SDHW is a system in which the domestic water can bee heated both by solar collectors and by an auxiliary energy supply system. The auxiliary energy supply system heats up the hot-water tank from the top and the water volume heated by the auxiliary energy supply system is fitted to the hot water consumption and consumption pattern. In periods with a large hot-water demand the volume is large, in periods with a small hot-water demand the volume is small. Different system designs have been tested over longer periods in a laboratory. Numeric models of the systems have been developed. The models are verified with measured temperatures and energy quantities. In order to verify the good test results in the laboratory two of the smart SDHW systems have been installed in practice. Measurements will bee carried out for a period of one year.