i was thinking about that q earlier today. essentially you're giving the character the ability to completely negate a caster in lane, as you can be burning huge portions of their mana often with no repercussions thanks to your character being energy-based. i think that almost makes it an imperative for them to be IN a lane rather than out - max the q, shut down annie/brand/malz/orianna/sona/any primary laning mage. you'd also negate a character like cho, amumu, and galio in-lane because their mana regen already sucks, and early-game their abilities cost so much (amumu, for example, his toss takes up roughly a quarter of his mana per throw at level 1 and gets worse as you level it).
i think if they were a mana-based character, that q would be OK. make it 80 or something relatively high, to use sparingly to empty mana reserves before an attack or gank. but not energy-based or cooldown-based, because then it's simply too much mana lost. alternatively, keep it energy, but make it like 25 or 30s cooldown, flat. no scaling. that would encourage it used in ganks as opposed to in-lane because you'd only be using it once anyways, at the start of the gank, whereas in-lane you'd use it when it was up but then not have it available when an opportunity arose later. maybe give it a passive that's not available during cooldown as well?