This simple system is effective enough to require backup heat only after cloudy days in December, January, and February. My only backup heat is a large stone fireplace, modeled after the high thermal mass Russian and European designs. Mine also provides domestic hot water. My space and water heating bills are near zero. Passive solar systems are simple in concept and use, have few or no moving parts, require little or no maintenance, and require no external energy input.
Passive systems collect and transport heat by non-mechanical means.
Active systems employ hardware and mechanical systems to collect and store heat, often using some outside energy source such as electricity for fans and pumps. The greenhouse effect is most commonly demonstrated by leaving a parked car in the sun with the windows rolled up. We all know how hot that can get because it lacks storage. In a passive solar building, your windows are your collectors, your walls and floor are your absorbers and storage. Water and phase change materials can also provide storage.