If you are preparing to have a PV array installed on or at your home for the purpose of providing your home with that electricity and you want to get state offered incentives to help pay for it, then it will need a permit and henceforth an inspection and if the inspector is familiar with residential PV systems he will be looking for the UL stamp of approval not just on the PV modules but everything in the system. I cannot say that it is a "must have" but I do know it will raise the eyebrows of an inspector experienced with residential PV systems if he doesn't see the UL stamp of approval. Best bet if you are going this route, check with your local inspections office first.
Here is a place you can start your search.
http://photovoltaics.sandia.gov/Across the top of the page, look for "search" and type in something like "UL listed PV modules".
Don't forget about your homes insurance policy, they might have something to say about the PV system as well.
