I have 400W of solar panels, a 1oooW inverter, an 80W deep freeze and one 50Ah car battery.
To run the deep freeze puts such a strain on the little car battery and inverter at startup.
My plan was to use mains to start the deep freeze running, and then using the DPDT to switch over to the solar driven battery powered inverter.
(I know the answer is to have a mains connected inverter, but I don't have one).
I did wire up a DPDT relay, and tried it out on a 100W incandescent lamp swoping from 220V mains ac to another 220V mains ac.
It worked fine for the lamp with barely a flicker.
I have been too scared to try it on the inverter/fridge/mains setup although I am sure it would work. But the cost of inverters being astronomical, I have postponed testing it indefinitely.
That is why I was suggesting that it would be similar to swoping between two inverters.