The summer attic; it's hot. Really hot. 140 degrees Fahrenheit, sometimes hotter. Boilers attached to your radiators make water cooler than this for heating. So getting that heat out of your attic should prevent some of it from heating your house and should decrease your air conditioning bills, right?
It depends. As with all real situations, there are circumstances under which an attic fan works and others under which it increases rather than decreases your bills. So, before you spend $500 wiring up a fan, let's review.
First, air leakage. If your attic leaks (and most likely it does - unless you had it air sealed already, it does), it lets warm air out of your house in winter and leaks hot air into your house in summer (this is a major reason why the upstairs gets hot in summer). Add the attic fan, and you draw air-conditioned air out of the house in summer, and the fan will very likely increase your bills. So, you MUST air seal the attic floor plate first (in fact, all energy efficiency improvements begin here...even when you swap those windows that didn't need swapping...). All the recessed lights, the ceiling fans, the drywall seams, framing seams, the attic hatch, pipe runs to the basement...the works. It must be sealed. Period.
Next, ductwork. If you have ductwork in your attic, and it isn't air sealed, you have the same problem. So the ducts and air handler MUST be air sealed at the seams and filter box. The register boots (the elbows) must be caulked to the drywall for an airtight system.
Finally, air source. The fan has to get air from somewhere. You must have inlets: soffit vents, gable vents, some sort of piped intake...something. Otherwise, the fan will pull air from the house (no: standard intake vents are not enough to offset the draw across a leaky attic floor). How big should the vents be? They (the statutory they) say 1 square foot per 300 square feet of floor area. But, of course the attic fan draws more air than that...how much draw...how big of a fan?
There are various ideas about this. The bottom line is a minimum of 400 CFM (cubic feet per minute), probably closer to 800-1200 CFM needed to reduce attic temperature more than a few degrees (remember, you are fighting massive amounts solar radiation here). The manufacturer should specific the minimum surface area of intake required for the fan to have proper flow.
So, recap. You may get some comfort from an attic fan, if it is big enough. And it has to have proper air intakes. And first you have to air seal and insulate your attic, in order for the fan to work for you at all.
In other words, eating your peas keeps your ice cream cold. No free lunch in nature or physics: short the hustle, seal and insulate your attic first.