As you play missions, you will occasionally find a discarded pizza. These will refill your energy back to 100%. If you do not have an empty Pizza Box, it will automatically increase your energy to 100% immediately. BUT, if you do have a Pizza Box, your pizza will go in there instead and you can play it whenever you want (like when you are running low on energy). If you notice, there is an orange box with a "+" in it next to your energy gauge. If you click on that, it will show your Pizza Box and if you have pizza in it, there will be a button under it that says "Use Now" (otherwise it will say "Empty Buy More"). When you get low on energy, just click the "Use Now" button and your energy gauge will go back to 100% so that you can continue playing. Then, hopefully, you will find another discarded pizza while you are playing to refill your box.