And the car batteries can supply peak demand instead of having a blackout.

I hope that doesn't mean I plug in my car at 80%, hoping to top it off for a long trip, but since everyone else is averaging 60% and there's peak demand, it decides to drain the battery and generously put a dollar in my account.

It would most likely be opt-in, as a form of this is already a thing in some place - you can choose to have the power company reduce the power of your cooling/heating during peaks in demand in return for a discount on your power bill.

Try building some generators? Renewable energy is cheap and a perfect match for charging cars, just adjust the charging current to match current generation.

Rolling blackouts are being experienced because the delta between high day usage and low night usage is too great to invest in upgraded peak-capable infrastructure. Charging vehicles at night will actually _improve_ the situation.