Remember, hot air rises. And remember for every degree that you cool your attic that makes the living space down stairs cooler and easier to keep cool. Remember that attic ventilation is a system and combination of many other ingredients.
Roof ridge vents are great, they dispell heat that accums at the top, these are good, but also you need a way to invite fresh air from outside to come in and circulate around and then out. Soffet vents are also good if the holes are big enough and un-obstructed and able to draw. Drawing like for a fireplace is just as important for an attic, the heat or hot air leaving creates a void or vacume and fresh air needs to fill that void easily.
Wind turbines do a good job, but soo many are needed to actually create a draw. Then when the wind is not blowing, do they actually do any work? I say no. Attic power therm, controled fans are good, IF you have openings from which to draw fresh air.
Case in point, I went to a home last month that had been recently sided with alum siding. The house had hip roofs all around. Previous to siding there were soffett vents all around. The homeowner did not check up or just did not know, but the siding people sided over all the vents.The attic was closed or sealed up. The customer began to notice that the compressor was staying on much longer than before. Then one day he entered the attic from inside the house for storage of something and noticed an unbearable amount of heat.
I told him what had happened, he was shocked. I suggested to find the soffett vents and open them up again, then I suggested to add a large therm power suction fan, the attic was again alive and breathing and his compressor was not staying on as much and more like before.
For all skeptics, he had the compressor checked, added more attic insulation, and checked filters. So, a hot attic was the culprit.