I stripped a water heater down to just the tank, painted it black, put it in an insulated box also painted black inside, and covered the box with glass.
The first picture shows the box during construction where I packed insulation in between the box framing. This insulation came from the original water heater and so did the sheet metal I sealed it in with. The wood was mostly scrap laying around the place, so everything was free!
The second picture shows the box with insulation in place, and the
third picture shows the inside of the box painted black.
The picture missing here (because it's still in my camera which I forgot because we left for town in a hurry and so much for casual, stress-free life in the country, but anyway...) shows the completed heater sitting out in the open under the sun warming up our shower water for free.
Just painting a tank black would provide hot water. Placing it in an insulated box will keep the water hot overnight, and will extend the season well into the autumn.
