The heat from waste water goes mainly into the pipework close to the house and in cold climates it prevents freezing.
For water heating the best choice is evacuated tubes even in countries like Ireland that have only one day of clear skies in six. In winter water supply is usually much colder which means the panels work with weaker sun. The gain in the coldest six months can be around 50% of the summer gain. In any case houses should have an efficient heating system for winter that also tops up the water temperature.
Heat pump water heaters that use internal house air as a heat source are really only for summer use. In winter the heat has to be replaced from other sources that could heat the water directly instead.
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