Ban has always been an exception, capable of fighting commandments despite Meliodas and the others at the period being stronger and destroyed by Galand, it is always because of his abilities. Now, I believe Ban’s power level is only around 200k but his newfound powers are just too overpowered and perfect against the Demon King particularly Inversion.
Ban post purgatory is truly strong, not as strong as the Demon King but he is incredibly strong. He was the only one able to damage the Demon King, neither King and Mael were able to do so. His body is able to resist to the worst conditions and his speed now is top tier, like I said weeks ago the reason why Nakaba took off his immortality before this fight is that Ban would have been too overpowered if he had his Immortality.
Ban and Demon King Power Level Comparison- Nanatsu no Taizai
I know power scaling in this manga is not that great, but I still want to speculate and predict the power level of Ban and Demon King.
Demon King Power Level
The share of the DK power was the commandment given upon the Ten Commandments. He divided his power in half and bestowed it to 10 commandments. From Estarossas’ powerup from getting in the commandment of Truth, he got an addition of around 35K to his Power Level. Using that and multiplying it by 10 then 2, the Demon King should be around 700K Power Level alone without powering up.
Ban Power Level:
Ban got a surprisingly huge boost, his physicals seem insane and I’m sure his limits with snatch have developed. I believe Ban’s current power level is around 200K without the snatch. When Ban was battling with Galand he could steal 7000-8000 from Galand and stated that is the max power level which he can steal. So he can absorb about 3 times his primary power. In other words, if he simply has right now 200k he can reasonably absorb at least +400k making his power level around 600k.
I do believe that one way or the other the Demon King is not quite as strong as he could be in other scenarios. Supposably the Demon King power level is so mighty he can’t be in the human world for too long. But if his aim is to roam the human realm then he can’t ever use his actual power. So chances are his power right now is just about as high as it can be without it causing harm to the world as a whole. Unless he doesn’t worry about that and his intention is to be in the human world while causing all that destruction.