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This skill is great for energy recovery if you are fighting close to the frontlines. It not only recovers up to 18 energy for a mere cost of 5 energy (13 energy gain), it also drains up to 9 energy from an opponent.
Energy Drain provides slightly less energy over time than Mantra of Recall, but without the Mantra's awkward delayed energy gain and vulnerability to being shattered. Energy Drain also permits the use of extended enchantment duration items as well as items that improve inspiration spell recharge time.
Keep in mind that draining a target with 0 energy will not restore your own energy: you only gain energy for the amount of energy you are actually able to drain.
Energy Gain
This presumes Energy Drain is cast as often as possible (activation + recharge time) before any activation/recharge time modifiers.