what he ^ said, with a 5000w inverter you dont need a battery, you need a bank of batteries to get an ammount of time
https://www.google.com/search?q=all...nq2AWExsHxDw&ved=0CEYQ_AUoAQ&biw=1198&bih=702. it might even be more practical to "invert" your thinking , and use way more efficient items, and/or things that run slower lower and or on lower voltages/amperages during an emergency. Like 12V fridges, and led or CFL lights, some backup systems have seperate lighting for the backups, way back when (~1960) they would have 12v incan bulbs and 12v seperate wiring to deal with that, today we have many efficient things that can replace the hogs.
compressor motors are very hard to start, but 5k whould pull it off.
I calculate ~300Amps at a low 12V would be needed for ~3500w pull that is some serious storage, and some serious cables. assume that somehow you could pull the 3500w from a 12V battery, it would be like continually trying to start a 8 cylinder gas hog, as the starter motors for big engines use that ammount of power at thier peak.
Gells. marine batts are lead acid, and unless you have 10,000USD to build a specialised ni-mhy or lithium system (like a 'lectric car) lead acid is the only way to do that kind of power for any lenght of time.
You will basically be having a storage system that is Very Very similar to a solar system that has battery storage , and that would be a good place to get information.
Yes the lead acid batteries when charging (but mostly when overcharging) let off hydrogen gas, and (i am pretty sure) let out the oxygen too (seperate the water) , and that gas mix loves to recombine given any ignition source, which can include pilot lights and sparks.
A "battery Bank System" has some sort of ventilation and safety for that aspect of it to insure that does not happen. or is in a box outside, many different methods, and some people use like no methods
to insure that aspect of it. It is possible to put them indoors with some of the venting methods used, either crazy enough, or having it worked out well enough. many garages are not very tightly sealed, and making ducting and venting, or even sparkless fan venting is all possible and done to solution the gas issue. which still leaves the fact that these things are full of acid.