Passive solar energy systems are very efficient in that they don't require any machinery or electrical devices.
These systems can be used to heat water, houses and other buildings.
Houses designed to use a passive solar energy system generally have large windows facing the South, to receive the most sunlight possible, with overhangs to block some sunlight in the summer.
Any costs in creating this system would come from altering the design of a conventional home with materials to absorb and distribute solar heat.
Passive Solar Energy