Also you can just wire a second breaker panel next to the main panel. On the second panel put a heavy wire and plug, more than actaully needed to power your circuits. Now take the wires out of the main panel for those circuits you'll want to run from the inverter and wire those into the second panel.
Place inverter outlets near second panel and wire a heavy outlet from the main panel.
Now it is as simple as deciding what you want to use as power source, grid or inverter, and plug the second panel to that source as you want it.
Or you could just wire several 120V outlets under your panel and put heavy duty plugs on the crcuit wires, plug them in like an extension cord to either the grid power outlets or the inverter.
I have done and do both ways.
Also I have done that extension cord trick, plug into inverter and wall outlet. While not a good thing to do for other reasons, really if main power breaker is off then nothing should be backfed into the grid. Should be no danger to line workers. CHECK to be certain. Things could be wired other ways than normal.
Wiring may vary! All I have done like that, the main breaker either flows power or breaks the flow, in or out does not matter, it's either on or off. Since both hots of a 240v grid (2 120V lines) goes through the main 240V breaker then if breaker is tripped OFF no power can flow either way through it.
I have seen some boxes wired direct and there was no main breaker though!
Or once I found a 120V breaker for one leg and other wired direct to the box. The guy used the breaker side for heavy 120V shop stuff and other 120v leg wired direct went to low power house stuff.
I have 4 breaker panels here, 1 at meter, 1 at well, 1 in house, 1 in kitchen. Grid starts at meter and runs in that order to where power is used, through each box in series in that order. I you can shut off the grid at some point before it enters the box like I can, then you can shut of the grid, test for no grid power at the box. Wire the extension cord trick or other stuff and check again to be sure the grid side of box is still dead when main breaker is off in that box.
That's another trick we used to use on a portable office. Grid power when on location, a generator when moving. We used a simple 2 breaker setup. One 240V breaker was the genny, another the grid. The breaker simple act as a main breaker and power only flowed to the box through the on that was on. The breakers switch was connected together so only one could be on at a time. Turning one on forced the other one off.
The point here is basically that is what back feeding through an outlet does, the breaker for that 120V outlet is simply a main breaker then for the power going into the panel on that line. Once in the panel all the breakers for that leg of the box will be live. The other leg of the box will be dead still. Mostly I have done this when grid power was not available and I needed a few outlets or light working in various rooms.
If you turn off that breaker your feeding into, then all the rest of the circuits on that leg should go dead. All outlets on that same circuit you are plugged into will still be live direct from the inverter still though.
This has been a good way for me to find what all is on a 120V breaker when I don't know the wiring of a house or such.
I make sure there is no grid power!!!! Normally there is not or I could just turn off the breakers one at a time and see what don't work.
So I wire to back feed an outlet with an inverter, maker sure ALL breakers are off.
Check every outlet and switch to see what is live and mark them. Then plug into a dead outlet and see what is live now. Continue untill all 120V breakers are known.
To find out what breaker is for the outlet you plugged into if you don't know, turn on several other 120V breakers, then simply flip on a breaker till you find one that turns on other lights that don't work when it was off, that's the breaker for the circuit you plugged into. 2 Breakers will operate one circuit this way, the one acting as your main, and the one for the other circuit with the light, so have 4 or so breakers on and the one that shuts off your other 3 is your feed circuit from the inverter.
Of course easiest to just shut them all off and test with a meter to find the live one, but if a meter is not handy that works.