Attic fans are designed to cool overheated attics; they draw in cooler outside air (through the roof vents) while pushing overheated air to the outside of the attic… And since attic temperatures can easily reach exceed 130ºF (54ºC) during the summer, that seems a good idea.
But contrary to manufacturers' claims and the many positive customer reviews involving this type of fans, their use is very controversial, to say the least. They may not provide any significant thermal improvements.
Attic fans vs. whole house fans
Whole-house fans are often called attic fans, because they are installed in the attic floor. But contrary to true attic fans, whose goal is to cool the attic, whole-house fans are larger devices aimed to cool the entire house. See: Whole-house fans guide.
Criticism on powered attic fans
Energy Star program does not exactly oppose attic fans; but it doesn't recommend them either.
It just points out their disadvantages in improperly ventilated attics, and the potential problems that come from using attic fans in poorly-sealed attics: the fans can pull cooler air from the living space, making AC equipment work harder and increasing the cooling bills instead of reducing them.
On the other hand, it states that there is no Energy Star label for attic fans «because they use more energy than alternative attic cooling methods such as passive ventilation».
FSEC claims that «attics with natural ventilation and R-19 ceiling insulation do not need powered vent fans. Such fans cost more to operate than they save, so they are not recommended», while Dominion points out the unintended consequences of using attic fans…
Curiously, customer reviews do not agree with these views. As expected, ratings vary a lot, but there are some brands and models with very good customer ratings. See, for instance, in Amazon: Attic fans
Attic fans should only be considered as a last resort, in overheated attics. They are easy to use and install (solar powered fans), and relatively inexpensive, but not what buyers expect.
You may lower the attic temperatures by using reflective roofing materials, light colored roofs, reflective roof coatings, or by installing or improving a soffit-ridge or other attic ventilation system, but the only reliable means of avoiding warm air to escape into the living space or heat transfer to the rooms under the attic is a very airtight attic, with very high levels of insulation.