Energy in Shadow Fight 2 is needed to fight. There is a maximum of 5 energy units, and the player uses 1 energy to enter any fight (except for Ascension, Raids and Challenger fights). Fighting General's Mercenaries needs one energy, but doesn't consume the energy. The player will gain 1 energy every 10 minutes, but they can gain energy quicker by either watching a video to gain 1 unit, or paying 5 Gems to refill all the energy. The player can also purchase unlimited energy, which comes at a price of $19.99.
In Shadow Fight 2 Special Edition, the player has unlimited energy. The energy bar is replaced with the "Unlimited Energy" icon (see picture above).