So what? The idea is not to eliminate the grid but just to maximise the benefit of the solar PV array and battery combination during the year. In my case a battery will all but eliminate my import of electricity for the spring summer and autumn months as even on a dull day I can put 600W into the battery for a few hours and that is then enough to boil a 3kW kettle, heat a dishwasher or washing machine and run a hob and oven for a meal without importing electricity.
On a sunny day like today it will charge to over 70% and that is enough to run my average evening load at this time of year (300W) for 10 hrs.
Okay I know it won't do this in mid December but it should cut my current dependence on the grid of 40% of my electricity to 20%. In fact if I am sensible I think I can do better than this.
The big advantage may come when fixed price tariffs end and standard electricity is at 30p/kWh and there are still too few EVs or electrolysis to H2 businesses to take up all the off peak supplies, then buying in 6kWh at 5p displaces having to buy at full peak rates during the day. I don't think the ROI on the battery gets interesting (in the absence of PV) until the peak rate gets to around 30p/kWh or the house uses more electricity than I.
Now if only I could get a Thermal Electric Generator with a 5% Heat to electric yield then that 400W would top it up nicely in deepest winter.