Normal Galactus is just as powerful as the top tier Celestails One Above All, Arishem, as where if he absorbes all the power from his world ship Taa II, he would be even with Eternity. Even Eternity stated in comics trying ti find it, but still he said Galactus at full potentail can be my equal, if it's comes from Eternity that pretty much seals the deal how powerful Galactus can be.
Some important things to remember about Galactus is that he is a fource of nature/reality and merely does what he must to survive and play his role in maintaining the existence of the universe. He doesn't seek after higher levels of power normally, because that's not necessary for him. He feeds on enough planets to maintain his power and keep Abraxas in check, no more and no less. However, given forewarning for a battle that would threaten him and thus the Universe, it would be likely that he feed heavily. Also, remember that back in Secret Wars (pre-ret) Doom, through trickery and subtly steals the power Galactus was supposed to receive from consuming his entire worldship, a ship the size of the solar system, ship that Reed Richards thought might be the greatest single source of energy in the Universe. Doom, with this level of power was strong enough to assault a pre-retcon beyonder in his own realm and then use a trick to steal his power. I would say thats stinking powerful.