My wife uses a portable O₂ concentrator and the combination of 2x6v batteries and solar is working very well for us. The specs on the machine were a bit disturbing; but seem to indicate a startup surge, rather than continual usage.
10 hrs usage with fridge and furnace included draws the batteries down to 78%,which the solar panel recharges easily. We haven't tried 2 days w/rain, yet; but it looks doable.
I had Reace use 8 gauge wire to the 12v outlet closest to the battery. It is the only outlet that will power the O₂ concentrator. It seems to draw a great deal of power during startup and not so much running.