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Using sun energy for water heating purposes is known thing, maybe not so popular.

https://en.wikipedia.org/wiki/Solar_thermal_collector

Main problem that places with good solar output rarely require mass usage of hot water (except for cooking), and places that has cold climate not so good for solar power.

And even in sunny places solar energy not really reliable, even for cooking (it also couldn't produce fire), so you need to rely on other energy sources.