Planning a Backup solar system for my Main solar system! Any ideas welcome

Since I am planning to setup two buildings (cottage and workshop) with solar panels, I am running into a challenge, how to provide maximum PV input to my
2 x EG4 18kpv (not yet connected) in parallel as well as my already existing Growatt 12 kW with 24 x 300 W Canadian Solar panels in 4s3p mode.
I know, that is just numbers and consideration of the specs of the units I have, but another challenge comes along. I want to have a backup solar system, which helps me avoid the automatic start of the propane generator so often.

My setup:
Both electrical panels of the buildings are connected with a 90 amp 3 conductor cable.
Main building:
All 24 solar panels are now connected to a Growatt 12 kW inverter.
I am using the existing combiner boxes of he former Magnum system with the bird house. I just had to shut off one PV-string (breaker) in the combiner box, to stay in the spec of the 250 VOC on each PV input ot the Growatt. So right now 2 strings with 4 solar panels are not connected but mounted on the roof. The system runs now with 24 panels.

Workshop:
So my plan is to wait for my 2 x EG4 18kpv inverter to use them for my main building as well as the workshop. When they are installed in parallel, they shall be connected to enough 445 Watt Canadian Solar PV panels, so that most of the PV inputs of both EG4 18kpv will be working. There are enough solar panels on site, because I made a good deal on a skid.

The Challenge:
The Growatt 12 kW shall be a backup source, only working, when the EG4 18kpv system might have a problem, providing both buildings with power.
I know, that this might be a luxurious problem to have a backup solar system, but wouldn't that make sense to any standby generator solution?
I am not planning to detach the Generac generator, because my dryer, instant water heater as well as the range are running on propane.

Does anyone have a similar or different approach and knows how to connect the backup system, so that the backup source could be automatically be used (like now with the automatic generator start), when the main system needs it?

Without a diagram, initial thought: Sounds like an Automatic Transfer Switch would be the answer. Treat the pair of 18kpv (not kW) inverters like utility power and the Growatt like a generator.

I can't see it makes sense to have a backup PV system to your regular solar setup. Generator yes to carry you when for reasons of weather or equipment failure you are forced to supplement.

Sometimes you could run into a situation (having air bnb guests, open gatherings) where you might be lucky, when a completely separate solar system provides you instant power. Also, when a EG4 unit fails, has to be repaired and I can't get a quick fix. So I am more thinking of backup system than of backup unit or device.

Mattb4 said:

I can't see it makes sense to have a backup PV system to your regular solar setup. Generator yes to carry you when for reasons of weather or equipment failure you are forced to supplement.

Click to expand...

BentleyJ said:

Without a diagram, initial thought: Sounds like an Automatic Transfer Switch would be the answer. Treat the pair of 18kpv (not kW) inverters like utility power and the Growatt like a generator.

Click to expand...

So the AC input of the EG4 would handle the AC output of the Growatt? I kinda like this idea, when it works.

Dr.Bannock said:

So the AC input of the EG4 would handle the AC output of the Growatt? I kinda like this idea, when it works.

Click to expand...

Actually I wasn't suggesting that at all. If I understood the first post you are looking a separate source of power if maintenance is needed on the EG4. An ATS takes 2 separate sources and routes one or the other to the breaker panel. In this scenario it would act like a maintenance bypass switch so the EG4 system can be electrically isolated.

I add a drawing of the situation. The Generac generator is connected also to a switch, so that the Growatt can get either power from the generator or from the duel fuel mobil generator.

When I see it now, I think I would like everything except the EG4s working as a backup with a manual or automatic switch, if possible.
There are 2 battery racks of 6 EG4 48 Volt/100Ah each.
6 x EG4 LL
6 x EG4 LifePower