I think his actual capacity is pretty low since he's still a bit green... like, maybe he can only keep the electricity up for some minutes or so before having to recharge. The catch is, he CAN recharge, so why not do it?
If he had chosen another power he wouldn't even be able to do so and would have to take a rest... that's why i think he chose a great hatsu, not everyone can recharge aura like he does, like, hisoka can't just chew gun to get more aura back and silva can't blow himself up, killua chose something that can be turned back to the source material.