How big a battery would we need in order to be completely self-sufficient?

Exactly. Haven’t read all details of the link,but soI react your comment, and have immersed myself a bit in this earlier.

You need to change your way of thinking and energy usage. Start with your daily energy supply and then change your energy consumption pattern to day time use Then, with for example a dynamic energy contract or if you can spare solar energy, buy or store cheap electricity in your storage ( battery ). The energy management system ( charge / uncharge and which cells) is very important.

Als…realize that battery life is tied to charge cycles and need replacing like every 10 years when talking about the better quality Lithium battery . Soda systems could and maybe used in parallel, if you want more storage, safety and longevity.

It is yet all quite expansive, though imo having a half day reserve ( like 5 -10 kwh) battery, would already create more independence.