I think the difference is how abrupt the change is. On a cloudy day the solar production is more or less reduced for the entire time. It's not like a switch which the eclipse will bring. You can get thundershowers of course, complete with dark skies, but those are local and scattered.probably about the same as a wide spread cloudy day, no?
The total eclipse will last for about 2 minutes. Not a heck of alot to prepare for. A tree falling on the lines or a reckless Model S driver taking out a utility pole is likely to interrupt power for longer.It will be interesting to see how the grid in California will be able to handle the sudden drop in solar power when the eclipse begins and the sudden return of power when the eclipse is over. I'm currently remodeling a home with a Tesla Powerwall 2. We checked its emergency backup operation yesterday in preparation for the eclipse, I think we will be OK.