I agree that tooltip is accurate enough, the thing that threw me off is I could swear it wasn't draining all my AP earlier in the game. I must have been mistaken. I'm now regretting taking Thermodynamicity feat, because at least at this level, it's kind of a waste. I have two fire-based abilities so far, one with a fairly long cooldown and one that drains all my AP, so I can't do much chaining of fire/cold spells. Maybe it's designed that way so you can't abuse it, but it's pretty lackluster as a result for now.