Every solar attic fan gets a score from 0 to 100% built from five factors, which maps onto the 5-point rating shown on the site (a 0% score would be 1.5 stars; a perfect score, 5). Each factor is scored against fixed reference points, not against the other products in the list, so adding a new model never moves another model’s rating. Fans are grouped by mount, roof and gable, on the same scales.
| Factor | Measures | Weight |
|---|---|---|
| Value | price per CFM of rated airflow | 25% |
| Airflow | rated CFM, up to 45 per panel watt | 15% |
| Panel | solar panel watts, the power behind the airflow claim | 20% |
| Features | thermostat, AC backup and a separate or tilting panel | 15% |
| Warranty | warranty length | 25% |
Where the numbers come from
Airflow, panel wattage, attic area, controls, AC backup and warranty come from the maker’s spec sheet, manual or product page, or from the maker’s own copy on a retail listing when that is all that’s published. Prices are checked at Amazon, the maker’s own store and home improvement retailers, and the lowest is used.
The factors
Value is the current price divided by the rated airflow, on a log scale, so halving the price per CFM always counts the same. $0.60 per CFM scores zero and $0.10 full marks. It uses the credited airflow described next.
Airflow is the maker’s rated airflow: 600 CFM scores zero and 2,500 CFM full marks. Makers rarely say how they measured it, and some budget fans claim far more air than their panel can drive, so airflow is credited only up to 45 CFM per watt of panel. Brand-name fans publish 25 to 50 CFM per watt; a 32 W fan claiming 2,400 CFM is credited with 1,440, and the factor is marked estimated.
Panel is the solar panel’s rating, the power behind the airflow claim: 10 W scores zero and 60 W full marks.
Features is 40% for a thermostat (or thermostat and humidistat) that turns the fan on and off, 30% for AC backup, an adapter included or sold for the fan that runs it from house power at night and on cloudy days, and 30% for a panel that tilts or mounts apart from the fan to face the sun.
Warranty is the maker’s warranty: 1 year scores zero and 25 years full marks. A lifetime warranty gets full marks. When the panel and the motor are covered for different terms, the shorter one is used.
What isn’t scored
Rated attic area, fan blade diameter, motor type and weight are listed but not scored.
Missing data
If a figure isn’t published, that factor gets the neutral middle score (50) and the product page marks it as estimated. In features, a feature that isn’t published counts as absent, and the factor is neutral only when none of the three is published. The exception is warranty: a maker that doesn’t publish one gets no warranty counted, so that factor scores zero, the same as the shortest warranty on the scale, and is marked estimated.
Ties and discontinued models
Products are ranked by score within each group. Discontinued models stay listed but rank below every current model and can be hidden with the “Hide discontinued” filter.