One of my co-workers (I'm Australian) has 500 kilowatt-hours of storage at home...which is wild. Much more common is the 10-20 kilowatt-hours of domestic storage for a house.
More details please, do they have a website that explains their setup?
Are they a hoarder of old car batteries and the like?
My dad buys lead acids written off from storm damage to solar systems (The whole system gets replaced under insurance even if the batteries are just a bit worn) and then sells them to preppers in the middle of nowhere. For a while he had above 300KW/h of storage, basically completely off grid with few shutdowns. It was kind of nuts. His house did burn down, but it was arson.
Does he have a saltwater aquarium, or any other hobby that can make use of it? If not, I can highly recommend that he get into it, if he's into that kind of overkill :)
That's ~8 used EV batteries. Each cost less than 10k, maybe 6-8k AUD.
If you know your way around high voltage DC, got a tractor and appropriate emulator - not exactly difficult or super expensive to pull off.
Granted it's pretty uncommon setup as grid batteries themselves are pretty cheap too and used EV battery is simply too large for home user, too much hassle, liability, etc to save like $2-3k.
What is their fire suppression setup like??? Granted I guess they could be doing pumped hydro storage lol