In the 100% renewable grid, electricity actually will be in surplus a good part of the time, because so much excess capacity would be installed. This is not the case now, so you can't use the current frequency at which curtailment occurs as some sort of baseline.
Yes, you'd need excess storage so it doesn't run out. Fortunately hydrogen storage is cheap. This is another argument for hydrogen over batteries.
You can run the numbers and see that in a hypothetical system for providing steady power in Germany, including hydrogen storage can cut the total cost nearly in half (subject to assumptions, of course.) Doing it with just wind, solar, and batteries ends up being far more expensive.
