Passive solar energy refers to collection systems that do not involve the input of other forms of energy to increase the effectiveness of the collection system. This usually refers to pumps, blowers and automatic systems to aim collectors, lenses or mirrors. Many systems are still classified as passive even if they involve electrical and mechanical systems to close insulating shutters or move shades. Passive solar systems are considered direct systems although sometimes they involve convective flow which technically is a conversion of heat into mechanical energy.

Passive solar systems make use of architectural elements in buildings to take advantage of natural cycles of sunlight and other elements in order to reduce the costs of building heating and cooling, without using mechanical elements. If mechanical elements are used, then the system is termed active solar.

In the strictest sense, passive solar has these main elements:

• Building orientation: The building is oriented as close to a north-south-east-west axis as possible, usually with the long axis of the building running east-west for maximum exposure towards the south (in the northern hemisphere; towards the north in the southern hemisphere).
• Window placement: Windows are minimized on the north side of the building, which in a northern temperate zone receives no direct sunlight from the fall equinox to the spring equinox. Windows on the south side (in the northern hemisphere) are made large in order to receive maximal sunlight during the winter.
• Shade: Overhang projections are provided over southern, and sometimes eastern and western, windows to shade them during summer months. Because the sun is at a lower angle in the sky during winter days than summer days, such structures can provide shade during the summer yet allow the sun to shine in through windows in the winter. Landscaping may also play a part in this: deciduous trees may be planted on the window side. In summer, these trees will shade the house, cooling it, while in winter when they do not have leaves they will not greatly obscure the influx of sunlight. Trellises with plants that only grow in summer can provide shade in the summer while allowing light through the winter. The trellis pieces can be angled to increase winter light.
• Thermal mass: Dense building materials are used where the winter sun will shine on them inside the windows, to capture the sun's heat and re-radiate it through the night. A trombe wall is one such structure. Another more efficient form places concrete blocks under the main slab, using the holes in the blocks as ducts to treble the heat exchange area (the below-grade ducts must be drained). Passive solar buildings often use massive materials such as stone, concrete, straw bales, and adobe.
• Daylighting: Windows and internal reflecting areas are placed so that they can maximize the sun's light for interior lighting during the day.

See also:

• Insulation
• Weatherization
• Earth sheltering
• Energy-efficient landscaping
• Environmental design
• Active solar
• Renewable energy
• Autonomous building
• Straw-bale construction