On a south facing roof, large flat boxes about 10 - 15 cms deep, painted black on the insides, with a glass face. The air inside the boxes heats up. The boxes are connected via plastic tubes, and on down into a cellar. A return pipe comes up from the cellar to feed already warmed air back into the boxes. The air is circulated by a single fan in the tubes, powered by a single solar cell.

The insulated cellar is filled with large granite rocks stacked to leave airspace around them. The rocks slowly heat up from the warmed air passed through them.

At night, when it is colder, the tube fan is switched to pull up warm air from the cellar, and a vent is switched to redirect the warmed air into the house, and isolate the roof tubes so warm air is only circulating from house to cellar.

You can't be me, I'm taken