The question is never “how big a battery should I get”. It is “what do you want still working at two in the morning during an outage”.
Write that list down. For most households it is the fridge, the router, a handful of sockets and — out here — the well pump. That list draws far less than people assume, and a single 13.5 kWh unit will carry it comfortably through a night and well into the next day.
Where batteries get expensive is when the list quietly grows to include air conditioning and an electric range. Both are legitimate, but they change the size of the system by a factor of two or three, and you should make that choice deliberately rather than discover it in the quote.