Are you using Solar Thermal System? What will you do when there is no solar light,in rainy,in snowy,in cloudy,in night....? Usually, electrical heating will be installed in the system,or maybe,A boiler will be installed bewteen solar collector and indoor water pipe. But all above equipments take you much money,as they themself are using much energy-electricity or coal,or gas....
Now,there is a revolutionary equipment to instead of electrical heating or boiler to heat water when there is no solar light. See the below photo,which looks like an unit of air conditioner outdoor.

Installation is very simple,what you should do is to connect solar collector and water pipe,and then put plug into socket electrical.(Notice,the equipment is powered by electricity,but consumption of electricity for this equipment is very small,is about 25% electricity,compared with electrical water heater).

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