Exhaustion 101.
The basics:
-You lose 10 energy from your maximum.
-You don't lose energy from your current energy. So, if you cast an Exhaustion spell at 30/30 energy you'll have 25/20 energy.
-You can spend that excess energy you just can't regenerate it.
-You cannot cast a spell with a cost more than your maximum energy.
-The effects of exhaustion wear off at a rate of 1 maximum energy per 3 seconds, the equivalent of a pip. So, at 3 seconds you'll have -9 maximum energy, at 6 -8, at 9 -7, and so on until it disappears completely at 30 seconds (There seem to be fractional values so that's all an equivalent).
-Exhaustion should disappear when you die.
The more advanced stuff. Exhaustion is something that's typically found on Elementalist skills, I can't think of a skill from another list offhand that has it, and it helps to limit those skills to primary Elementalists. Exhaustion only hurts when it starts to cut into your ability to cast spells, that's when you dip below the casting cost of a skill. Elementalists have Energy Storage which increases their maximum energy so it takes them longer to reach that point. For most characters an Exhaustion every 15~30 seconds is fine. Two or three is going to cripple you. But for an Elementalist you can easily absorb the effects of four or five exhaustions before you start to feel any ill-effects. Skills that cost Exhaustion and have recycles of around 30 seconds, like Ether Prodigy, are pretty muchpenalty free. It's when you get into spammable spells, like Gale or Obsidian Flame, that you'll run into trouble.