It's not a reliability number. For comparison, right now 31% of California's energy comes from renewable sources (according to wikipedia).
What 99.9% means is that they'll keep some traditional power generation mechanisms, like gas turbines, around to fill in when the renewable supply is not sufficient. That could be because there isn't enough sun or because there is unusually high demand. The power isn't going to go out.
What 99.9% means is that they'll keep some traditional power generation mechanisms, like gas turbines, around to fill in when the renewable supply is not sufficient. That could be because there isn't enough sun or because there is unusually high demand. The power isn't going to go out.