Anyone have experience with solar attic fans? How effective are they?

I have an old house (100 years) and it has no soffit (intake) vents for the attic. I have a moisture/humidity problem in the attic that I need to resolve. Currently, I have 2 gables vents and nothing else. It was recommended by an energy auditor that I install a powered vent on the roof near the ridge and he recommended solar. Do these things have the power to move enough air? I asked about how the humidity will be dealt with when the sun goes down and he told me that it won't be as much of a problem at night (funny, I didn't know that humidity goes away in the evening). Anyone have any EXPERIENCE with solar attic fans? They effective?