So long as a character does not use any abilities which would otherwise drain the suit, sleeping & being out in the sun should keep a character in the positives. They just won't recharge as fast as someone who gets plenty of rest & eats regularly!

If characters begin to starve themselves their suit will stop gaining back charge and gradually slip into the negatives the longer a character forgoes nourishment. If they do not suffer any adverse effects naturally from not eating however it will have little to no effect on suit power.