That 12 day event was a regional event in Germany, there was still lots of wind in the North Sea. And sun & wind didn't drop to 0, it dropped below 10%.
Dunkelflaute's are a German phenomenon: the standard pattern in most of the world is that the high pressure systems that suppress winds are generally sunny. And the high elevation areas in Germany are sunny during a dunkelflaute.
IOW, batteries aren't the only answer required to cover a dunkelflaute.
Hydrogen is one possibility for a 100% grid to cover Dunkelflauten and seasonal leveling (the former for wind, the latter for solar). To see the effects on overall cost per MWh, explore at https://model.energy/
Dunkelflaute's are a German phenomenon: the standard pattern in most of the world is that the high pressure systems that suppress winds are generally sunny. And the high elevation areas in Germany are sunny during a dunkelflaute.
IOW, batteries aren't the only answer required to cover a dunkelflaute.