A Tesla Powerwall 2 stores up to 13.7kWh nominal. It also has features like setting minimum retention charge levels for unexpected power cuts and stormwatch that ensures the battery is automatically recharged at or near capacity before bad weather arrives. It’s not cheap but I guess you “pays your money and takes your chances.” It works without any issues when the grid is down and I can also run the house from a generator if needed.
Interesting: we have PV panels, plus a 2.5kw Pythontech (SP?) battery. When the power goes out it is pointless and I've no idea if and when it is working. The PV panels have provided about 3500kwh a year, we use about that per day in winter, dropping to about 10% of that in summer. If we turn off non essentials we can cut consumption to about 1.5kwh, so a PowerWall 2 would give us 10 or so hours of essential power to run the gas boiler, internet, fridge and a few lights. So time to look at costs....
