Something like this (I now see similar one listed for $10), and an entire house is the protected loads. Entire garage with arc welder etc. is on grid, not protected. (but manual interlocked breaker lets me power garage for loads such as extra fridge; I would turn off or not operate heavy loads.)
The color might be a little different from the actual product due to color display of different monitors. Installation mode is 35mm DIN guide rail installation. when you give us a positive one. Used for cutting nylon fabric, nylon rope, plastic rope, woven belt, etc.
www.ebay.com
I have a 6 awg run to my pool house, which has its own breaker panel. Eventually I plan to pull 2 awg as well and have interlocked breakers in the panel, so I can have 70A straight from grid (main panel) or 100A from battery inverter. But pulling 200' of heavy wire is tough, need a cable tugger to do that.
So for now I have this interlocked dual DPDT transfer switch. One side goes to main panel fed by grid. Other side goes to output of battery inverter. (With a load-shed relay to disconnect in case of deep battery discharge, because my AC coupled system needs to keep grid-tie inverters connected.) The 6 awg wire to pool house is on other side of transfer switch, and I used split bolts to connect output of the two interlocked DPDT (because there is no busbar joining them.)
(Like figure B, then wrapped in rubber tape and vinyl tape)
I have this transfer switch inside a rain tight square-D box I gutted and mounted DIN rail in. Companies offer busbars used in combiner boxes to parallel breakers which might work, but I'd need something to work with interleaved breakers (two hots on each phase.) The following combiner has fingers for three adjacent SPST breakers, shows two installed, but wouldn't exactly do what I needed:
The problem with how it is set up now is that I may not notice a power failure, and battery will try to run A/C, laundry, electric heat after sun goes down, which won't be for long. I want to add relays to shed those excessive loads. In the case of A/C, it'll be a small relay in the thermostat circuit. Or, once I pull 2 awg, move some loads (electric heat) to grid-only not battery inverter. Goal is to run A/C and laundry on surplus PV while sun is up during grid failures, but disconnect them at night so battery is there for balance of smaller loads in house.
With your transfer switch panel, you keep refrigerator on battery inverter and A/C on grid (looks like the panel has individual 120V circuits, no 240V circuits?) If a power failure comes you can manually switch over A/C knowing you need to turn it off when the sun gets low.