I agree with Glenn. The best way is natural ventilation. Save on not buying solar and improve your natural venting, less to go wrong.
Folks are just never in the attic on a hot day,and don't see whne the fan quits.
On another note, I would also improve my insulation in the attic, to help on heating and cooling costs.
Check out www.nuwool.com
for a really good solution to that.