There is not a best solar powered attic fan. Research has shown that attic fans are a waste of money and will never pay for themselves.
Typically these fans are oversized and there is not enough intake air vents to create a balanced system. Due to this, a negative pressure is created in the attic so it then sucks your conditioned air (that you paid to cool) from your house into your attic.
- As air leaves your house out the attic, new hot humid air from outside then comes in to replace the air that just left.
- Your air conditioner will now have to cool and dehumidify the new unconditioned air that was brought it.
Even if the attic fan did work, it would only be helpful during the summer and do nothing to help in the winter.
Your money would be much more wisely spent air sealing and insulating your attic and you would see the benefits year round.
For more information:
Read Ian MacLeod's Q&A "What is the safest and most effective insulation for our attic crawl space?"
Also, read "How effective are the different attic ventilation methods? I read mixed reviews, and many have a commercial bias." a Q&A answered by Danny Kelly.