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I wonder of the drawback as well. I can see it needs more fiddling - the keg needs to be filled then the warm water should be poured into the plastic barrel - from my and my family's part so it's less convenient than just turning the tap and lo warm water.
Other systems along this line could be simple to work with but most likely more labor-intensive to set it up: creating a fixed sunny place where the heating barrel stands then build the plumbing to the faucet but it still could be built relatively cheap and for greater volume of water, so maybe shower could be covered too. Or if it's an "official" thing, with the solar panels on the rooftops which creating warm water and not electricity, it will cost much which only can be cost efficient on the long run.