Isaac is probably trying to find a way to maintain his memories through the loops by being put into the inventory (either that or he's trying to study it from the inside).
I'm assuming he's either lying about his lack of energy or taking another gamble by actually using a lot of it in order to win Skelly's favor (the goddess sapping his power thing is probably complete BS).
Now the question is whether he's doing it for his curiosity and goal or if he's doing it to get a one up on Skelly (it could actually go either way).