@MJH-
I've been working on this idea for almost a year. My case is "very" close to yours, as follows
1. Parallel Inverter Generators, converted to Natural Gas (actually tri-fuel now) (derated ~ 3000W/6000W [sing/dual], running)
2. Powerwall (battery bank) for: Active Sump Pump Backup, Passive home backup in evenings (or when I want to treat neighbors).
Took 5 months for Inverter Generators to be delivered, so it was painful to say the least.
Worse than that the parallel kit wasn't applicable to home backup, 240v only setup.
I've had my gas meter upgraded and ran 3/4" ID pipe to garage with dual QC fitting manifold. Just ran my 1st test with both gensets yesterday, but still need to convert 2nd geny to NG.
I wanted inverter generators to have best THD (<3%) for sensitive electronics.
I like parallel option so I can run one and be quiet (or save gas if needed) - neighbors will like me ;--)
Unique in my gensets is they're parallel, AND 240V capable, so I'm able to feed both legs of my MAIN which feeds my sub panel (~50 breakers).
I went open frame so it was easier to convert to natural gas.
My plan is to accept I can't backup 240V oven or AC (fine for me living in MI with 2000 SqFt basement)
I also like setup is mostly portable so if I move this can follow me - for the most part.
I approached this that if it was easy (or common practice) to plug in an alternate backup generator, then just do the same with a battery setup. The key is I think is to treat it as a temporary setup (not as many requirements to meet?).
Two typical options in home backup power connecting are either (1) use an interlock being able to power whole panel, or (2) use essentials box converting only limited switches to dual power option. I'm considering something that's a bit of both (more on that below).
A key is to understand how much backup power is needed. I used a product called Sense which monitors home power (and device) usage. This was important to me in deciding generator size and design of battery backup size. Typical US home uses about 30KWH per day, and about 10KWH per night, as Sense helped me confirm.
To "actively" have my sump pump backed up by using essential panel is a typical setup I believe. My sump worst case is pumping 2 times per minute, or 1200 GPH ! (thanks to detailed data from Sense). I figure this can be done using a hybrid inverter / battery bank setup, AKA super UPS system. The AC "passes through" the inverter then immediately converts DC to AC if power is dropped, like a typical UPS.
I actually have 3 small battery UPS batteries (~ $100 ea) for 3 satellite routers and IT stuff are covered short term.
So, the question for me is do I battery backup my sump pump for a few minutes, a few hours, or 10KWH for the night?
The only automatic (active) part of my setup would be sump pump, which is perfect if I'm gone. The trick is to route power, manually, and backfeed my panel same was as generators. I came up with some ~$100 transfer switches (more reliable that ATS). I even thought of routing via a cord something like old fashioned switchboard operators...
Attached is my plan at this point - still a WIP
Below is showing 2 Inverter Generators (before vs after NG conversion + wheel kit).