I did some man maths to decide battery size.
Outside of my car charging and in addition to solar production, my average daily consumption is 8.4kwh.
I am actually delighted that my battery is covering my home electricity consumption in Dec/Jan, this is when I rely on the grid the most because of very limited solar production.
A second battery on my current usage would very rarely be used.
I am starting to consider a 2nd Zappi tho