Finding the right size of a solar powered attic fan for your space is essential to keep your home breathing properly. Remember: A fan too powerful or too weak will not address your ventilation problems adequately.
As a matter of fact, excessive fan power (high CFM rating) won’t deliver proper ventilation. In fact, it can even draw out air-conditioned air from the living space, which will overwork your A/C unit.
Yet, an attic fan too weak (low CFM rating) will do little to cool the air in your attic. It will not also fend off the build-up of moisture that causes mold and fungus. Your ventilation system should always be in balance. This means the amount of exhaust ventilation should never exceed the amount of intake ventilation.
Here’s our 3-step process to determine what size solar powered attic fan (SPAF) is needed for your space:
1. Calculate the Square Footage of Your Attic Space
The U.S. Federal Housing authority recommends a minimum of at least 1 square foot of attic ventilation for every 300 square feet of attic floor space. To determine the size of your attic, multiply the width by the length of the attic floor in feet.
- Example (20’ wide by 50’ long attic): 20’ x 50’ = 1,000 sq. ft. attic space
2. Determine the Ideal Size of the Solar Attic Fan
The minimum size needed for an attic fan is 700 CFM for every 1000 square feet of attic space. To know what size of attic fan your space requires, multiply the square feet of attic space by 0.7 to get the minimum number of cubic feet of air per minute. This final number gives the CFM required for the attic fan for that particular attic.
- Example: 1,000 sq. ft. attic x 0.7 = 700 CFM minimum fan rating
- Add an additional 20% (CFM x 1.20) if you have a steep roof, and 15% (CFM x 1.15) for a dark roof.
3. Finally, Determine How Many SPAFs are Needed
Use the charts below as a general guideline to knowing the number of Solar Attic Fans needed for residential installations.
| Venting Capacity | CFM Output | Solar Panel Size | Number of Fans/Model |
| 500 – 750 sq. ft. attic space | 250 – 400 CFM | 20WSPF-FLEX | (1-2 ) 20WSPF-FLEX fans or (1) 18W-INT fan |
| 500 – 1000 sq. ft. attic space | 850 CFM | 18W-INT | (2-3) 20WSPF-FLEX fans or (1-2) 18W-INT fan or (1) 40W-INT fan |
| 1000 – 1500 sq. ft. attic space | 1,100 CFM | 40W-INT | (3-4) 20WSPF-FLEX fans or(2-3) 18W-INT fans or (1-2) 40W-INT fans |
Note: Mostly, manufacturers rate their fans based only on the fan motor peak CFM output. Not the average CFM performance during all direct sunlight hours. In line with that, iSolar offers solar attic fans ranging from 250 CFM to 1125 CFM (hourly average in direct sunlight).
Important: Contractor pricing (volume discounts from retail price) is available starting at a quantity of 10 units (any combination of models). Also, orders of more than 10 units enjoy free shipping.
