Our building is heated by oil. This method obviously isn't as eco-friendly as sun panels, but it still does the job just fine. It's a really reliable source of energy. In those 20 odd years, in which i have lived in this appartement, i've never had to experience a freezing cold night or anything of that sort.
Here is a picture of how a regular oil-heater works.
*I wasn't able to take a picture of the heater itself, because it's in a seperate room to which the tenants have no acces.