9,000' up - it's not the extra cold that would bother me, it's the extra snow that's going to cover up my solar panels.
We store ours at 5400'.
Personally, I don't think once a month would be sufficient to keep them topped off. Maybe someone else with actual experience will chime in.
I've gone solar now and just head out the to the storage lot after a snowstorm to clean the panels off.
Taking them out, if not too much hassle, and putting them on a trickle charger is one way to guarantee they'll stay fresh.