I currently have wired power vent in my Attic, it Quit working. Not sure
what size it is, but I wanted to replace it with Solar version. I am not sure how
to calculate what size fan to replace it with or if one fan is even enough,

The 20 watt solar fan I found so far moves 1300 CFM, How do I calculate
what size fan I need,

My attic does have ridge vents as well and get feed from the sofits

Is it hard to calculate the volume of the attic? What do you use to determine
the volume of air you need to exchange to keep attic cooler.

I read on the web if you have ridge you really do not need power fan

I can tell hugh difference in the temp of the attic w.o the power fan. I so not even know the size fan the builder used. I am sure its the lowest CFM

Thanks George