Energy Storage

Energy Storage is an Elementalist primary attribute meaning that a character with an Elementalist for their primary class can put points into this attribute.

Guild Wars Description
"For each rank of Energy Storage, your maximum energy increases by 3. Several skills, related to gaining Health or Energy, become more effective with a higher Energy Storage."

General Description
Like most primary attributes, having at least a few points in the attribute is heavily recommended. A few points in Energy Storage means you can have 12 more energy, which translates to being able to cast that one Fireball when you really need it. The value of having more energy is pretty self-explanatory, but if that wasn't enough to make you want to put points into Energy Storage, take a look at the skills that are linked to it.

This attribute includes an Elementalists only two healing skills: Aura of Restoration and Ether Renewal. Aura of Restoration is an enchantment which heals your health a percentage of the energy cost of any spells you cast while you have it active. Generally speaking, this is the only real healing spell an Elementalist has, since Ether Renewal heals such a small amount in comparison, and is elite, forcing you to wait until near the end of the game to aquire it. Ether Renewal heals you energy and health for every enchantment you have active on you, each time you cast a spell, an incredible benefit if you have more than one enchantment on you and you have a cheap, quick spell to cast.

Lastly is the odd, elite skill Ether Prodigy. This skill gives you energy regeneration of +4 (equivalent to 4/3 points of energy per second), but when it ends, you take 2 damage for every point of energy you still have. This may seem like no big deal, but when you have 10 points in Energy Storage you have a total of 60 energy, so you'll take 120 damage if you simply use this to heal your mana. The best use of this is to simply cast all your expensive Fire Magic skills in quick succession until this effect ends. If you do it right, you'll take almost no damage, which is the exact opposite of what your foes have taken.