This isn’t rocket science. While some manufactures size their fans for a certain number of square feet, we know that is just a guideline. Square footage measures the approximate size of the floor of the attic and doesn’t take into consideration the volume or cubic feet of air in the attic. Then there’s the climate (Arizona vs Maine) and the construction of the building (brick vs wood) and even the color of the roof (dark vs light). After all that, the real answer is “more ventilation is generally better”.
The SRS-850 (850cfm in full sunlight) is specified for up to 1,200 sq ft. This unit has an 11 watt solar panel. We put the SRS-850 over a garage, in a small attic or as part of a gang of fans in a large attic.
The SRS-1050 (1,050cfm in full sunlight) is effective for attics up to 1,400 sq ft. This unit has a 15 watt solar panel and is our work-horse fan; the one that we put on most single-family dwellings. We sometimes use two of these if the attic is especially voluminous or has several sections that don’t connect well.
The SRS-1250 (1,250cfm in full sunlight) is what we recommend for large attics or for commercial applications. This unit has a 20 watt solar panel and may be the most efficient of the SunRise Solar fans.
Solar Attic Fans operate best on a South or West facing roof. If that roof would be the front of your house, SunRise Solar makes a remote solar collector. That means that the smaller solar collector panel could be mounted on the visible part of the roof but the fan itself could be mounted almost anywhere it might not be as obvious.