If Superman is solar-powered (he draws his super powers from the yellow sun of the Earth), then how does he retain his powers at night?
Is he like a battery, where he stores excess during the day and runs on that at night? If so, then what happens if he does too much at night? Will he burn out and become like the rest of us until the sun comes up again? What happens if he goes on vacation and just lies around in the sun all day? Does he build up so much power that he explodes?