But adding solar heating isn't always all that easy, especially when that home wasn't designed with solar in mind.
When most people talk about solar heated homes, they're referring to passive solar. In a nutshell, passive solar works by the principle that dark colors absorb light. Since light is energy, the law of the conservation of energy tells us that the light must be converted to some other form of energy. This naturally occurs by converting the sunlight into heat. So, as long as a home is designed with enough windows, a good absorber and sufficient thermal mass, a passive solar heating system will work.
Unfortunately, not all homes lend themselves to passive solar heating. While many can be adapted to receive some benefit from passive solar, there are some with designs or locations that make it cost-prohibitive to modify them for passive solar.