High-end skyfather, at best, imo. Purely theoretical ofcourse, as he's not done anything beyond topping Umar's best against the legion of Mindless Ones.
I'm not sure what the current explanation is for his energy source, but at one-time it was stated as being a separate universe/dimension, though for decades now they have depicted him amping from gamma in 616 and from nuclear explosions and various other sources too.
He's also been a conduit for universal energies during Heroes Reborn, his body can handle that scale of energy through-put, it seems.
There is some - tenuous - supporting evidence for the idea that he could operate on a "universal scale", from the suggestion that he is an "omega level" threat and the idea that he is a mutant (can't recall where that originated). The whole Beyonder thing is probably the biggest booster for that.
Also, we've never seen him approaching a definite limit yet, and we've seen some pretty impressive feats.