House battery backup (no solar) and generator implementation

[Kairus]

Hey folks, I've been thinking about putting together a battery bank for emergency power for a while and I'm getting closer to pulling the trigger. My goal is to to put together a system that would easily allow me to switch back and forth between battery power and generator power during a prolonged grid outage, say after a hurricane. While the battery bank is depleted I would run the generator to power the house and charge the batteries. I envision that during a multi-day outage I would run the generator for most of the day and switch to the battery bank at night.

Even during shorter outages it would be nice to have power without having to roll the generator out. Right this second if the power went out I would be very hesitant to hook up the generator. It would take me probably 15 minutes to get the generator set up (rolling it out to the backyard, connecting the propane tanks, fetching the inlet cord, etc). With my luck the power would be back before I got the generator running. Grid power here is extremely stable aside from a flicker or two during a really bad thunderstorm.

I already have a generator hook up installed and a 120/240v split phase inverter generator. My main panel has a 30-amp double pole breaker with an interlock that connects via 8 gauge wire to a 30-amp inlet on the side of my house. I'd like to leverage this existing wiring to fit a battery bank in-between, that way the interlock on my panel functions the same regardless of battery or generator power and I don't have to complicate my main panel set up further.

For the battery bank I'm planning on using two 12v 300Ah LiFePO4 batteries in series with a Sigineer APC3024D 24v 3000w 120/240v split phase inverter charger. This inverter has a 30-amp automatic transfer switch built in. Down the line I would plan to expand this setup with another two batteries in parallel.

My idea is modify the existing wiring:

[30-amp generator inlet] -----> [Main Panel]

To:

[30-amp generator inlet] -----> [Inverter AC input] -----> [Inverter AC output] -----> [Main Panel]

The inverter's ATS would handle switching between inverter power and generator power. I have a few issues/concerns though:

1. This inverter only has one AC input so if I wanted to charge up the battery bank from grid power I would need to disconnect the generator connection from the inverter's single AC input and then plug in my house's grid power to the AC input. This isn't a major issue since my panel has an interlock so I would already be doing some manual tasks such as flipping breakers on/off. I would prefer a setup where I didn't have to worry about this though due to my next (big) concern:
2. If I left the inverter's AC input hooked up to a house circuit, say after charging, and flipped the interlock to start running the house off the inverter, what would happen? The inverter's AC input would technically be hooked up to its AC output after flowing through the main panel. Would it cycle on/off trying to transfer the load and charge the batteries? I'd prefer to design a setup where this wouldn't be possible but I could still charge the battery from a house circuit. I'm sure I could put a big red label to remind me not to leave the AC input hooked up to the house's circuits but that's a bit hacky.
3. The inverter's AC input is 240v only which means I would not connect the neutral from my generator yet the inverter still outputs 120/240v split phase. That means even when the inverter is simply passing power through from my generator the power would be running through the transformer to spit out 120/240v. The inverter's manual says nothing about efficiency loss due to that, but I'm assuming there is some? I guess that's the convenience of leveraging the ATS but am I creating any other problems here?

Does anyone have a similar setup? I searched quite a bit and there wasn't too much discussion about how people wired their battery bank into their house. My other idea was to use a 3-pole double throw safety switch with line 1 being the generator, line 2 being the inverter, and the load being the interlocked 30-amp breaker on my main panel. The downside to this is that I would have to cut the load each time I switched between battery and generator. I don't think my loads would be too happy about that!

These are the loads I would expect to put on the battery bank:

• 9,000 BTU Mini Split, max load would be 800w but typically less as it scales down
• Refrigerator 1, pulls 115w after in-rush (measured with a kill-a-watt)
• Refrigerator 2, haven't measured but most likely about 150-175w after in-rush
• Server, modem, routers, PoE camera switch, about 180-200w
• Some LED lights, laptop charger, etc.

Somewhere around 1,200 running watts max. At night the mini split will probably pull 300w or so. I also have a hybrid heat pump water heater that pulls 700w in heat-pump only mode. I would maybe run this off the inverter, it all depends how often it runs. I do understand that I would only get 6-12 hours of runtime depending on how often these items ran.