Tesla Motors Club

I recently had 3 Powerwall 2's installed in a whole house configuration, added onto my existing solar. I've been tinkering with settings - Backup vs Self-powered - and have a couple of questions. Lately, I've been leaving it in self-powered at around 60% which allows the solar to run my house and charge the walls most of the way during the day while allowing the walls to run the house most of the night.

1. I noticed that the "Backup" option has a percentage reserve power setting available just like the Self-powered mode. I believe self-powered set at 100% is essentially the same as backup 100%. What's the difference then considering Backup has a percentage slider available also?

2. Scenario - grid power is shut off by the utility, as they are doing pre-emptively in our area in California. If while solar is running the house the batteries become fully charged, what then happens to the excess solar power generated - the power that usually goes back to the grid. The grid was "shut down" by the utility. Does that power back feed into the grid causing any kind of danger? Does it have someplace to go? It has to go somewhere. Or does the solar deactivate some of its production capability? Would like to know for safety.

Thanks,
Russ