You will find it isnt reaching the advertised 6k watts. Sorter like those genpower 8kva gensets on ebay for $800 wont even run a small box a/c or start a fridge.
If you want to use a welder with an inverter then your going to need 5k watts normal running - which will require a hard wired sized inverter.
Ever seen the genset/ inverter system most portable welders use? There big for a reason

doesn't matter what physical size the welder is it still takes the same amount of energy to start a spark and hold it (depending on wire size etc etc)