Advantages

Passive solar refers to means of using sunlight for energy without active mechanical systems (as contrasted to active solar). Such technologies convert sunlight into usable heat (water, air, and thermal mass), cause air-movement for ventilating, or store heat for future use, without the assistance of other energy sources. A solarium on the equator-side of a building is one common example.

Technologies that use a significant amount of conventional energy to power pumps or fans are classified as active solar technologies. Some passive systems use a small amount of conventional energy to control dampers, shutters, night insulation, and other devices that enhance solar energy collection, storage, use, and reduce undesirable heat transfer.

Passive solar technologies include direct and indirect solar gain for space heating, solar water heating systems based on the thermosiphon, use of thermal mass and phase-change materials for slowing indoor air temperature swings, solar cookers, the solar chimney for enhancing natural ventilation, and earth sheltering.

More widely, passive solar includes technologies such as the solar furnace and solar forge, but these typically require some external energy for aligning their concentrating mirrors or receivers, and historically have not proven to be practical or cost effective for wide-spread use. 'Low-grade' energy needs, such as space and water heating, have proven, over time, to be better applications for passive solar energy utilization

Savings

Passive solar can reduce heating bills by 50%. A monthly heating bill of $200 dollars, you could expect savings of $80-$120 per month. Passive solar features if included at the time of construction, or a remodeling effort, the effective net cost of improvements will be much lower and you will benefit immediately in your monthly cash-flow.
 
Sustainable Building Sourcebook

U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy: Passive Solar Heating, Cooling and Daylighting