You don't use batteries to cover that worst-case scenario.

If you want to eliminate Carbon emissions, you do. The electricity supply network is always built for overcapacity or else bad things happen with frequency and device damage

This says 5 hours to get to 99%:

https://reneweconomy.com.au/a-near-100-per-cent-renewables-g...

Windgas is actually pretty suitable for that last 1%. It may be 50% efficient to produce but it's easy to store large quantities of gas for long periods for those dark/still spells.

Counterintuitively that 1% of electricity generated from windgas will among be the most expensive "green" electricity but it will probably still be cheaper than nuclear power:

https://theecologist.org/2016/feb/17/wind-power-windgas-chea...