Wireing a 700 watt inverter

I have wired a 12 volt system in my home using 10 gauge wire and 250 volt 20 amp ac outlets I wish to make some sort of pigtail so that I can plug my inverter into an outlet and run ac devises.
Is there any such adapter for this purpose ?
I would remove the battery cables and replace with 8 gauge wire unless this is too small a wire.
OR is there a way to get 12 volts into the house on a larger
gauge wire and some how place battery lugs in an outlet box and then I can connect the inverter to the lugs with the alligator clamps
This would only be used if the power fails

Or is there a better way with maybe junction boxes,fuse panels etc.
Thanks
Rene