I've done powered and solar attic vents etc and read a lot of in-depth studies.
The solar fans are simply too weak to do anything really useful and the powered fans eat more electricity than they offset in air conditioning. Part of the problem is that most homes are not 100% sealed internally, so when you run a powered attic vent, you get negative pressure in the attic and wall spaces, and this can pull conditioned air from the interior of the home and cause infiltration of unconditioned exterior air...so you suffer a loss on the air you heated/cooled and then have some new air to heat/cool.
Good passive venting gives you some benefit, more insulation is a one time expense and very effective.
I had the same problem with my last two large multistory houses. One way to solve it was to use a wireless thermostat. It has a base unit that replaces your regular thermostat and a portable battery powered part that has the readout and controls. Put that upstairs and the higher temps there will kick off your a/c to rotate the air. It also had a programmable setting to allow the thermo to turn on the furnace fan only for 10-15 minutes once an hour. That pretty much kept the costs of balancing the air down and did a decent job of it.
Another solution is to have an HVAC guy run a secondary sensor from the upstairs to your thermo, but you'll probably need a new thermo that takes 2nd and 3rd sensors.
Whats worked the best for us is using the new hvac system we have installed to run on low continuously. We get about a 1 degree difference between the first and third floor, year round. I think this makes the heat and a/c run a little bit more but the comfort is worth it. Coupled with a high efficiency furnace and air conditioner, the extra work wont cost you much. Most modern hvac systems with a variable speed fan pull about 80-100 watts when running on low...comparable to a large room fan or air cleaner.
Drop in a 5" media or electronic air cleaner and for an extra 20 watts, you have continuously circulated and cleaned air.
We also have a small whole house fan that I installed right into the piece of drywall that drops into the attic opening in the master bedroom closet. When it gets cooler outside than it is inside, I turn on the fan to its low setting (about 1000cfm) and open a few windows in the warmer rooms. Shut it off when we go to bed. The hvac system in the meanwhile keeps circulating the fresh/cool air to all the rooms in the house.
I found that even small "regular" whole house fans pull an awful lot of air on their low settings. They also arent very well insulated and you can get a lot of heat/cooling loss through the louvers. The tamarack one I bought is perfect. It has a low volume low setting and little doors on top with about 4" of insulation on them that get opened by a little motor before operation and close when turned off.
Typical applications of residential ventilation products~by Tamarack Technologies, Inc.
I bought the HV series, made a little frame out of 2x4's for it to drop into, nailed that to the sheetrock drop-in for the attic opening, cut the sheetrock and mounted the grill, and ran the cord to a nearby wall outlet. In under an hour I had a removable whole house vent fan with a remote control. Its a little more work to permanently install it in the ceiling and find a 110v junction to wire it to, but not that much...
