Maybe I did quote it wrong because it's a 20amp circuit. Even if you go 12-volt DC in with 20amps that's not 400w. You figure it's an inverter going from 12v DC to 120v AC. Hmm, I wonder if 400w is just a nominal continuous load rating?
I haven't tried genuinely welding with this thing plugged in. I plugged it in, and struck a simple arc with it to see if it would power it on. I didn't think much about penetration or weld quality. I just ASSumed if it would strike an arc it would weld. Probably a bad ASSumption on my part.
It's academic for me, because I am not overlanding/trail hopping hard enough to break something for the most part. I don't pack a welder when we head out for a Sunday drive.