You don't absolutely need Overload just to finish him you know.
I know I don't need it to finish him. I needed it because I rely on it to get my magicka pool full again. Energy Overload is what I use to fill up my magicka. When you think it's gonna be there, and then you go to switch it on and it isn't, then you're pretty much SOL. If I knew it wasn't going to be there, I would have managed my magicka better.