Also when you can't see shit without electricity.
But yeah, it seems to happen at night more often than not. When it happened tonight it was still a little light out so I could at least navigate through my house and find a flashlight in case I needed one.
That was my first thought too, but then I thought it might be more likely that the writers/artists/whatever would give him a small dick, considering American senses of humor about small penises, and the character that is Jerry Smith.
Here's a screen cap of an x-ray of his dick and balls from the episode Interdimensional Cable 2, where he is given a choice to give up his penis for a great alien being (note, this is nothing not shown on air on AS):
Notice on the top right how his balls go farther than his dick? What the hell?