I recall traveling in Mexico years ago and every house seemed to have a black painted water tank on the roof. What a concept!
These are tinacos and they are very common in Mexico. They are there because water pressure is typically low in Mexico and it can also get turned off or to merely a dribble. So they keep a reservoir of water on the roof and have gravity to create water pressure. There is a pump to pump water up there. Richer places also have hidden underground water tanks. This typically has nothing to do with solar power.
My apartment complex in the Philippines also has one of these. But nothing to do with solar. There is a pump that gets turned on several times per day to pump water to the tinaco (I don't know the Filipino word for it).
In Mexico my Spanish teacher explained to me that one of his daily tasks, besides taking the garbage out, doing the dishes, etc., was to turn on the water pump at a non-peak hour and then it would get turned off by a sensor when it was full.