You can get a free water heating by the sun. In some countries you see on the roofs of all districts water barrels everywhere. Here it's something less obvious.
It helps if you paint the tanks black. A black object absorbs (almost) all wavelengths and (almost) not reflects them.
The light energy is absorbed by the black object.

zonnedoucheA camping shower is a black plastic bag with a nozzle on it. You fill it with water and hang it in the sun to warm up. Works fine. If there is sun.
If you have a garden hose lying in the sun a while you get the same effect. It works even better, because the mass is much smaller and warms faster. Works also better with a black garden hose.

Behind glass, sheltered from wind, in the sun it soon is pretty warm. A glass case around your roof barrel thus increases the effect, like tandem bottomless bottles around the hose.

Combine all this and you get a solar water heater: black, water-filled tubes behind glass.
The (isolated) barrel is usually placed inside.
The tubing that heats outside is a separate and closed circuit. A tube that spirals through the boiler, is as a heat exchanger to heat up the water in the barrel: the content of the second circuit.

If you want to get to work with this, then certainly foresee antifreeze, and a pressure relief valve. And keep in mind for the construction that hot water rises in account (and makes more volume).

You can instead of heating the boiler, also transport the heat to a radiator, and heat a room with it.

You can put the primary circuit, which heats up, instead of the sun, also lay in a compost pile. There it also will rise to 60 degrees C.
Since there is not a continuous operation, these systems are only suitable as an additional heating.