A homeowner aims to heat the domestic waste water with solar heat - in order to replace an electric boiler, for example, or as a supplement to a wood heating system. The homeowner has the choice between solar panels or a combination of grid-connected photovoltaics and heat pump. Which system is more favourable? A comparison of technologies established by the Center of Appropriate Technology and Social Ecology (Ökozentrum Langenbruck), together with INFRAS, the Interstate University of Applied Sciences of Technology (NTB Buchs) and the Institute for Solar Technology (SPF Rapperswil) has analysed this issue. The results show this: At present, the answer still depends on each particular case. The combination of grid-connected photovoltaics and heat pump, however, is already very competitive and is likely to become more and more attractive. If solar thermal power does not become more inexpensive in the near future, it is going to face more and more difficulties on the market soon.