Overcast

Effect: While exhausted, a character's maximum energy supply is reduced. Whenever a spell causing exhaustion is cast, the caster loses 10 maximum energy. The amount of maximum energy lost is shown at the right end of the energy bar as greyed out. Maximum energy lost to exhaustion will be restored at a rate of 1 point every 3 seconds.

Exhaustion does stack. If a character casts several spells that cause exhaustion in succession, his maximum energy supply will become severely reduced. Exhaustion can actually cause maximum energy to drop below zero. In this case, the deficit must be regained at 1 point per 3 seconds before actually regaining maximum energy.

Source: Exhaustion is usually only self-imposed by certain elementalist spells. The effect of exhaustion is triggered when an exhaustion-causing spell begins casting, not once the spell has been successfully cast, so cancelling casting or being interrupted will not prevent exhaustion. Glint has a hex that causes all the hexed character's spells to cause exhaustion.

Removal: Dying or changing zones will remove all exhaustion. The only other method is the natural regeneration of 1 maximum energy per 3 seconds.

Prevention: Glyph of Energy prevents your next spell cast from causing exhaustion.