Unlikely that wind drops in the north sea, bay of biscay and med at the same time.
Nuclear also goes stale once in a while, when plants are taken offline for maintenance.
Solar is predictable, it generates in the day (when demand is highest), not at night. With connected vehicles you have massive battery banks of power that can be bought back from people, and as you've used the excess energy when you have been producing more than the demand from those resources to create green hydrogen, you reverse the process.
Average yearly wind won't drop. But it may slower for a day or week. Unless you want to stop some parts of society for that time, you'll have issues.
Same for solar. It's not predicatable on day-by-day bases.
With using cars as battery banks, even if you skip the problem of having cars connected all the time, you'll need to redo grid in a big way. Both to fill simultaneously cars if cheap solar energy hits the grid and keep the grid going on a stale cloudy day.
Well we do that all the time at the moment, if it drops, prices increase, and your car sells power back (because you've got 300 miles in the battery but only need 30 tomorrow), green hydrogen halt production, bitcoin miners stop usage, grid level storage comes online
On the flip side when there's surplus that's used to make the hydrogen, to recharge the resovoirs, etc.
Many people feed back to the grid with their home solar etc, no difference to electric charging.
The problem is you can't plan for „this is rare“. Given how grid is getting more and more away from guaranteed sources, a once-in-few-years perfect storm may become a big event where shutting down hydrogen plants may be not enough to save the grid.
The other issue is, how much grid upgrades do you need to allow to keep it up from electric cars? Maybe even on green front it'd be better to have stable power generation with simplified grid. All those beefy substations don't come out of thin air.
Nuclear also goes stale once in a while, when plants are taken offline for maintenance.
Solar is predictable, it generates in the day (when demand is highest), not at night. With connected vehicles you have massive battery banks of power that can be bought back from people, and as you've used the excess energy when you have been producing more than the demand from those resources to create green hydrogen, you reverse the process.