Energy is the rogue's equivalent of the caster's mana and the warrior's rage. It is also used by the druid's 
[Cat Form] and death knight ghouls. Energy is required for the execution of many rogue abilities and Cat Form abilities, although standard melee attacks don't depend on it at all. By default the energy bar is full, with 100 points of energy. Energy regenerates at a fixed rate of 1 energy every decisecond (ie every one tenth of a second), in and out of combat.
Pool size and regeneration rate
The size of the energy pool and the regeneration rate are both constant - they do not increase with levels, and are not related to any statistics. However, there are various ways rogues can improve these.
- The consumable item
[Thistle Tea], which can be made by rogue cooks, provides 100 energy instantly on a five minute cooldown. At level 70, [Thistle Tea] gives 40 energy; at level 80, it restores 20 energy. 
[Pure Energy] and [Crimson Steer Energy Drink] are both quest rewards that when used restore energy.
Rogue options
- The [Vigor] talent permanently increases the energy pool by 10 points.
- The [Glyph of Vigor] increases the energy pool by an additional 10 points if you have the Vigor talent.
- The [Relentless Strikes] talent gives a 20% chance per combo point spent to restore 25 energy.
- The Vitality talent increases the energy regeneration rate by 25% when maxed.
- The [Combat Potency] talent regenerates energy on offhand attacks.
- The [Focused Attacks] talent regenerates energy on melee critical strikes.
- The [Adrenaline Rush] talent increases your energy regeneration rate by 100% for 15 seconds.
- The [Improved Sinister Strike] talent reduces the energy cost of [Sinister Strike].
- The [Arcane Torrent] racial ability of blood elves gives 15 energy.
Energy regeneration bonuses stack that way:
100% + 
[Vitality] + 
[Overkill] + 
[Adrenaline Rush] = max 255% = 25.5 energy per second
Equipment
- [Renataki's Charm of Trickery] On Use: Instantly increases your energy by 60 (reduced by 2 every level after level 60).
- Nightslayer Armor 5 set bonus: Increases your maximum energy by 10.
- Bonescythe Armor 4 set bonus: Your Backstab, Sinister Strike, and Hemorrhage critical hits cause you to regain 5 energy.
- Madcap's Outfit 5 set bonus: Reduces energy cost of your Eviscerate and Rupture Abilities by 5.
- Assassination Armor 4 set bonus: Reduces the energy cost of Eviscerate and Envenom by 10.
- Gladiator's Vestments 4 set bonus: Increases your maximum energy by 10. (Any combination of Season 1, 2, 3, and/or 4 will give the bonus.)
Druid options
- The Ferocity talent reduces the energy cost of Claw, [Rake], and [Mangle] by 5.
- The [Shredding Attacks] talent reduces the energy cost of [Shred] by 18.
- The [Omen of Clarity] talent grants a chance to reduce the energy cost of the next ability by 100%.
- Malorne Harness 2 set bonus: Your melee attacks in Cat Form have a chance to generate 20 additional energy.
- Thunderheart Harness 2 set bonus: Reduces the energy cost of your Mangle ability in Cat Form by 5.
Patch changes
- Patch 3.0.3 (2008-11-04): Regeneration went from 20 energy per tick to 1 energy per decisecond.
