Unlimited Energy pills are very effective tools that give you infinite energy for a certain time. You can use them by tapping. The energy you will gain depends on the level of the pill. You can check how long you would get by selecting the item and looking at the infobox. You can acquire Unlimited Energy pills from Chests and also purchase them from the Daily Deals.

Don't underestimate them; you can do quite a lot within the given time. Give it a try and see! :)