I am to late with this bit of information I am sure. I am surprised that no one mentioned it really. Its climate and average ambient high temperature for your region vs. PV module rated voltages. Yep! voltages, plural. There are three different voltages to one PV module.
1. the nominal voltage. for example 12
2. the open circuit voltage. 21
3. the working or voltage. 17
Its the last one, the working voltage that is an issue with your regional climates average ambient high temperature. The higher the temperature say, if its tropical for example, then the higher the working voltage should be. Something in the order of 18 vdc or even 23 vdc for example on a 12 vdc nominal system. In contrast if its a colder region like Northern Alaska one might get ample power from a 14 vdc or 16.9 vdc module on that same nominal 12 vdc system.
By why would you want to?
If the higher voltage works so well in the heat then its really going to crank out the power in the cold!
Some of the first "modern type" PV modules I ever bought where the Solarex MSX77's with a working voltage of 16.9. When I say modern type, I am referring to those that came with frames, j-boxes, and warranties of course.
On the particular system these where working on, during the summer they didn't have an easy time of recharging the battery up to 100% each sunny day and one could forget about equalizing. While during the winter, they excelled at charging and they could equalize. This is in the southeast region of the north American continent.
The gist of all is - the hotter a PV modules gets the lower its working voltage gets and this goes for all PV modules.
I am truly surprised that the PV industry is not incorporating some sort of thermal electronics to the back of PV modules in an effort to generate even more power from a single PV module. Of course this would drive the cost of an already over priced item even higher.http://www.powerchips.gi/index.shtml