Oh! I fogot to metion the solar powered systems.
Solar panels are good for heating your hotwater cylinder providing your cylinder is well insulated.They don't need direct sunlight just daylight or UV light is still good for converting light into electrical energy, But they are still very expensive and you need a lot of them.

Direct water heating by solar power also work but this is a little more complicated particlaly the water flow rate throught the heat exchanger tubes needs to carefully regulated i.e. to much flow and not enough heat, to little flow and the water can boil causing other problems. The Chinese are experts at making these systems, and they work very well. But also very ugly looking in your roof. The chinese don't seem to mind it looking ugly because they sell 10 million per year.