Q:

How do I add an exhaust fan to an old attic?

Our house's attic retains a great deal of heat during summer.  While we now have a good layer of insulation between the attic and the rest of the house I wonder if it would be worth it to add some kind of exhaust fan.  We live in a mid-Atlantic state so we have to contend with both 100°F summer days and 0°F winter nights.
The house still has the original slate roof so adding a typical roof-based system isn't an option.  There are three small circular windows that provide some air flow, but not much.  My thought was to install a fan in one of those windows to force a cross breeze during warm days.
Is this likely to save us any energy (given that the fan will have to draw some power)? Are there options for solar-powered small window exhaust fans out there?

A:

There are solar-powered gable fans - an Air Vent-brand fan can be picked up at Lowe's for just over $200:

The solar panel is on a cord, so it can be installed in the most sunny area while the fan sits in the gable.
From the Air Vent website:

The Gable-Mount Solar Vent is an exciting alternative to traditional attic ventilation. That's because it is powered by a solar panel that collects energy directly from the sun and converts it into electricity -- naturally -- to power a durable, high efficiency 24-volt DC motor. As a totally solar-powered solution, it's also environmentally friendly.
Painless Installation
  The best installation feature is that the fan requires no electrical hook-up. That means no electrician and no complicated wiring is required, making the installation of your attic fan fast and easy.
It mounts in the gable end of the home, behind a decorative shutter (not provided, see below). The solar panel comes with brackets that allow for mounting on the roof.
Smooth Operation
  The solar panel collects energy throughout the day under direct sunlight and converts it into "free" power to operate the fan -- providing ventilation during daylight hours, when it's needed most.
It creates up to 800 cubic feet of air movement per minute to remove uncomfortable heat in the summer and damaging moisture in the winter. And, the solar panel is wind-, hail- and impact-resistant to stand up to the elements.
Specifications

Moves up to 800 cubic feet of air per minute
Solar panel collects and delivers power under direct sunlight during the hottest part of the day without electrical cost
Wind-, hail- and impact-resistant solar panel
High efficiency, durable 24-volt DC motor
5-year limited warranty

(source: airvent.com) 
Model #SHT15W

Plastic
Sold in the color white only but it can be painted to match a home’s exterior
Automatically opens when the power fan is running and then closes when the fan is not running

A:

There are powered gable vents you can install (cut a hole thru the side of the attic, or use the already provided gable vent) and they're fairly inexpensive.  The best way to remove heat would be to use those small windows for makeup air and use a separate powered vent to exhaust the hot air.   
The previous poster has a good suggestion about setting up a PV panel to run the fan, but in order to get sufficient airflow through the attic you'll probably need a pretty large sized PV setup which can get prohibitively expensive.  I've been researching them for my motorhome - the rough thumb rule seems to be ~1000 dollars per 100 watts of solar power (I added in costs for batteries and inverter since your home doesn't already come equipped)  
Even a smallish AC powered gable vent fan is going to pull 400 watts - so you can see how this gets expensive.  There is a combination solar panel/attic vent kits which uses a DC motor but I'm not sure about the effectiveness. It seems to be rated fairly low in terms of air volume moved (800 CFM vs 1200-1600 for a 110v powered vent fan).
Exhausting your attic will definitely save you energy as you're removing a huge heatload from your air conditioning - the superheated air in your attic.

