It was forever ago that I looked into it, but if I remember right, solars were/are bugged so that they still don't work in rain. There was some thread that actually proved it, maybe I could find that again....
Nope, I think it got deleted, but I think it still is bugged. The isDaytime() function is based on skylight values, which are directly affected by rain/thunder. According to that old thread, the light values were low enough during rain/thunder to shut solars off. Anyone actually looked at a desert based solar during a rainstorm? I wouldn't be surprised if the sun indicator is blank...
I was under the impression this was intentional, and last I checked, solars work in deserts even when it's raining.