Like a high-end CNC or something?
If I ever own a house I'm going to buy an old VW Rabbit or MB 300D and use that motor as my backup - then combine it with those fancy windmills you see in Walmart parking lots AND solar. Between the tax breaks and the fun I'd have welding & building a quiet diesel backup solution I think it could be done much cheaper than buying and maintaining a refrigerator sized battery. That's pretty serious stuff.
I don't know where you are in the world but if you have a city nearby I'd talk to some low voltage union guys (they run the phones & cat5 cable in new office space). Outside that... batteries have a somewhat short lifespan.
In a big city I'd look for the low voltage guys. In a small town I'd look at the CPA's and tax attorneys. They will know who got what tax break and who might have what for sale. So hit up the local dive bar or the local country club. Outside that these things are rare and hard to come by.
FWIW, most people are looking proactive these days. Weather for example. A diesel motor at idle is really cheap to run/operate and it spins up quickly because it's already running & warm. Keeps your mechanic employed and avoids the "oh shit I hope it starts" syndrome". Plug into NOAA or other paid weather services (Bloomberg even has a feed) and take a proactive approach.
If you are planning on the freak accident actually happening you must be doing DOD work and have the budget to do it properly??
(or have an awesome grow room and just not want to share pics online)