Oh you can run some pretty big stuff from the cable that comes into your house. It's how you deal with it on the wall that makes the difference. From a domestic plug, 115 is around the biggest that you can plug in for a stick. But if you put in a separate breaker and a run a round plug, you can go considerably higher. Its limit is single phase really. Once you need three phase, you have to start getting all clever. I am no electrical engineer, but in terms of being safe and sensible, domestic supply will give you more than you'll ever need for a hobby welder. The trick is getting a breaker that is motor rated. Normal domestic mini circuit breakers are a fast trip. That's why they go when light bulb pops. it's not the running current that's the issue, it's when you strike the arc that the load comes on. I got a 20 amp D rated breaker IIRC. I did have a 30 amp std one and couldn't weld on full at all. Not the amps, it's the trip speed.
Hard to give advice exactly. 6mm steel you can weld with my 115 stick. But I wouldn't make a ship with it and sail the seven seas. Or make a tank from it. I guess what you are looking for in the main is the ability to join two pieces so that in effect there is no join at all. If you have 30mm steel, you'd need ability to penetrate 30mm or 15mm and go from both sides. See the point? But as most people don't go much about 10mm you'd need 10 or 5 respectively. Now, a 115 amp stick is not going to give you 5mm. Probably about 3mm with no root gap. So in effect you have a 10mm plate only joined for 6mm in places. My MIG is 180 amp. I'd have liked bigger, but really using .8mm wire, getting higher amperage will typically just melt the wire faster and not give that much more penetration. Tests have shown my set to deliver 7mm penetration so effectively I can fully join 14mm plate. I don't envisage using 14mm anything really. So welding 10mm sheet is well within my range. There is way much more to this than just blobbing weld everywhere and I don't know the half of it. But I do get the principles involved. Being able to weld sheet from one side only and have it appear on both sides of the sheet is a good sign. If you are serious about welding then forget the little disposable bottles. It's like having a EuroFighter with a jerry can sized fuel tank. You need around 10L /min flow when welding hard. It's highly addictive and if you like the off road stuff, in my view invaluable. Even a little buzzer like mine will open whole new Worlds. I may get another stick now that I have a proper supply, you can really bang down a good bead with a stick. I like 'em.
Chris