Millions of Israelis use 1950s tech rooftop solar devices for their hot water. Israel’s Tigi (see here previously) has developed cellulose triacetate insulation that can heat the water to 100 degrees Centigrade, even in freezing winter conditions. It has installed 40 international commercial systems.
https://nocamels.com/2022/11/honeycomb-twist-transforms-israels-1950s-solar-water-heater/