If you are seeing the grey button next to your energy drinks in the inbox, your current energy level has already reached its maximum. Once you run out of energy, you will always be able to replenish it from any location by opening one of the cans in your inbox.
_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-
Last Day on Earth: Survival Game GUIDE