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Natural Light's Solar Attic Fan is a simple and environmentally sensible solution that can save you money. Powered completely by free solar energy, this sleek and efficient vent is both compact and quiet. Fully operational right from the box, it installs easily, with no electrical wiring, no expensive electrician and no permits. And let's not forget powerful! Operating at a whopping 850 cfm, a single unit can fully vent up to 1200 square feet. Place it wherever you need improved circulation; attic, lofts, workshops, storage sheds, garages even barns. Need more power? The 20 watt model can vent up to 1200 cfm and an area of up to 1800 square feet.

Notes: 1) You must have adequate air intake for proper ventilation from the soffit vents or gable-end vents.
2) Place these fans away from any gable end vents.
3) Attic fans are not recommended in conjunction with ridge vents.

Not sure why unless they are worried about drawing in rain from roof vent? But this was copied from their ad at Solar Energy Store.