To determine what size solar attic fan(s) you need for your attic, you first need to know the size of your attic in square feet.
To determine the size of your attic, multiply the width by the length of the attic floor in feet. For a single story house, this is usually the same as the square footage of the house itself.
Next, multiply the square feet of attic space by 0.7 to get the minimum number of cubic feet of air per minute that the fan should be rated to move. 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. Attic fans come in all different shapes and sizes. Be sure to check the CFM rating of the product that you are interested in prior to installation to determine how many you need.