Overcast

Exhaustion reduces the character's maximum energy (similar to a Deep Wound's effect on your health bar). When exhaustion occurs, the caster loses 10 maximum energy. The amount of maximum energy lost is shown at the right end of the energy bar (in its default position) as grayed out. Maximum energy is recovered at a rate of 1 point of energy every 3 seconds.

Exhaustion stacks. A character repeatedly casting exhaustion spells will repeatedly reduce their maximum energy, lower and lower each time. Exhaustion can lower maximum energy below zero. In this case, the deficit is regained at 1 point per 3 seconds before regaining positive maximum energy.

Self-inflicted
Some elementalist skills exhaust the caster. Exhaustion triggers at the start of casting (like adrenaline or energy costs), not upon resolution, so the caster is exhausted even if the spell is cancelled or interrupted. One notable exception to this is the skill Invoke Lightning, which causes exhaustion upon completion of casting (if the caster is not enchanted).

The following elementalist skills cause exhaustion:
 * Energy Storage: Ether Prodigy
 * Air Magic: Chain Lightning, Gale, Invoke Lightning (if used while not enchanted), Mind Shock, Ride the Lightning, Shock
 * Earth Magic: Dragon's Stomp, Earthquake, Obsidian Flame
 * Fire Magic: Meteor, Meteor Shower, Mind Burn
 * Water Magic: Maelstrom, Mind Freeze
 * No Attribute: Second Wind

The following PvE-only skill also causes exhuastion on use:
 * Devourer Siege

External sources
The following skills can indirectly cause exhaustion:
 * causes all creatures in range to suffer double exhaustion when casting an exhausting skill.
 * inflicts exhaustion if it interrupts a spell.
 * causes the hexed foe's spells to inflict exhaustion.
 * (Monster Skill) causes the hexed foe's energy-based skills to inflict exhaustion.

The following environment effects can directly or indirectly cause exhaustion:
 * Aspect of Exhaustion inflicts exhaustion every 30 seconds.
 * Sheer Exhaustion causes all spells to inflict exhaustion.

Removal
Exhaustion naturally recovers one maximum energy every three seconds. Death does not remove exhaustion. Changing zones eliminates all exhaustion, though. There are currently no skills available which remove exhaustion.

Prevention

 * prevents your next spell cast from causing exhaustion.