Not needed, no payoff period.
Let's look at actual facts for attic fans.
I had a 120 volt, thermostatically controlled fan and the means to track what time of the day it came on and which days. The fan came on late morning, typically 10:30 to 11:00. More telling is the shut off. From 10:30 PM to 2:00 AM.
Next, consider solar panels on the fan are not binary. They generate power based upon the position of the sun, the energy generated determines the efficiency (speed) of the fan.
So, this means as you need a fan to expell the heat from your attic, which is heated by the hot roof long after solar noon your fan's speed is slowing and finally shutting down.
Finally, consider where the heat from your home comes from. It is a box with six sides, only one is impacted by a cool attic. Actually this has been measured. An UNINSULATED ceiling is responsible for 25 - 30% of your heat load. That is one reason your attic is the best insulated of all six sides of the box you call home.
For illustration let's say that your ceiling is not insulated and let's use a overly optimistic assumption that the solar fan will reduce your attic temp by 10% lower than it is already, that every month is August and your electric bill is 100% for electricity (no tax, absolutely no fees).
We can make this even more ridiculous and say that every bill is 250.00, that you need two solar fans to pull off this feat (@900.00 ea). All of this with the realization that about 55% of your bill is HVAC looks like this:
$250 (bill) x .55 (% of HVAC) x .3 (heat gained from ceiling) x .1 (savings from lower attic temp = 4.13 savings per month.
$1,800 (cost of fans) / $4.125 (monthly fan savings) = 436 months pay back. That's 36 years, four months after you remove all your insulation and devise a day for the sun to shine longer.
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