Energy Drain

Acquisition
Signet of Capture:
 * Prophecies
 * Sniik Hungrymind (Spearhead Peak/Witman's Folly)
 * Gambol Headrainer (Ice Floe)
 * Druul the Untamed (Dragon's Gullet (Only during quest The Titan Source)
 * Factions
 * Siska Scalewand (Archipelagos)
 * Nightfall
 * Rual, Stealer of Hope (The Ruptured Heart)

Energy Gain
This presumes Energy Drain is cast as often as possible (activation + recharge time) before any activation/recharge time modifiers.

Related articles

 * Gain/Lose energy skills quick reference