The state where the mega batteries are installed uses about ~32 GWh total a day. An 850 MWh doesn't even come close to covering their needs. Covering ~2.6% of their load is equivalent to keeping power on for about 40 minutes.
>Solar + battery is great in those locations, and basically no one is aruging using nuclear when solar and battery is viable for 365 days of the year. Australia has a fairly large desert where such assumption would be true.
Deserts actually make for the worst place to install solar. Dust / sand covering panels substantially lowers their efficiency (to near zero), and keeping them clean involves a lot of water (Can you guess where it's hardest to get water? Yep, a desert). And lastly, because the deserts are so far away from everyone else, there is significant transmission loss.
Solar + battery is great in those locations, and basically no one is aruging using nuclear when solar and battery is viable for 365 days of the year. Australia has a fairly large desert where such assumption would be true. Finland, Sweden, Germany, France, UK and most of Europe do not have reliable 365 days of sun each year. There aren't many investors spending their money to build solar + storage in northern Finland.