You car batterys capacity when its brand new is oversized for its job,
so you cant see its decreasing capacity. but decrease it does slowly
frm the day its manufactured.

battery backups for phone companies used HUGE BANKS of very low
capacity but long lived batteries. with solid lead plates holding
rosettes, would up thin solid lead. I have seen those batteries. last
30 years or more, but a big group would of been needed to start your
car. because heir capacity is so low...

mow take some marine batteries put them on a UPS. 12 volts in 120 out.
sounds great. but check the UPS manufacturers, the batteries used in
these last for just a few years at most. The UPS I use on my tv
satellite system is not quite 3 years old, run everything for a hour
and a half when new, now perhaps 20 minutes... battery is worn out..
costs nearly as much as a like sized brand new UPS I might add there
are tons of FREE UPS available nationwwide, all with bad batteries...

converting the 12 volts to 120 to run whatever isnt efficent, you lose
over 20% in the conversion process, transistor losses etc.

The BEST choice but pricey is a standby generator system. these monitor
the power line and any time theres a minute outage, the generator
starts, and switches over important loads in the home, sump pump,
fridge, furnace, etc and some lights. they use natural gas or propane
for fuel. easily cost 5 grand and up, I would like one some day.

the next best choice is a stand alone generator, 300 bucks and up, to
ru run one critical load at a time. I have 2 of these, a few thousand
watts, for real emergencies.

noisey, need gasoline, its not perfect but thy work. in this choice you
must be home to run it.

of course the best sump pump choice is fuel free. if your sump happens
to be higher than a low spot on your property, install a underground
pipe as a overflow. its always there and requires no real work after
the initial install.