Shoo! Shoo! Get that car battery out of your head. After a couple of deep discharges, the battery will be useless. A car battery has plates that are very close together and as a result it has a High Cranking Amp capacity or a low series resistance. It also means that supherization or when sulpher precipitates out of the battery acid, it shorts the plates together. Result, dead irrecoverable battery or commonly known as scrap.
A "deep discharge" battery cannot provide high cranking amps because the plates are further apart.
Long term outages - think propane or natural gas auto-start generator. Short term, think UPS.
You do have about 6 motors at 1 amps at 20 V, That's 180 watts. Not so fast. They need at least 3 A to start, so your UPS has to be able to deliver 3*30*6 or 540 VA for that brief time that they all start. So, my math is wrong, 2 pumps are supposed to be off. This is where VA and Watts differ in their meaning. Staggered starts can prevent this large starting load. I forgot the heater and stuff.
So, do we have a new design issue? The system has to run off of 12V?