Coffee has never affected my energy levels, but it has affected me in other ways.
I used to drink a lot of coffee (only black) everyday. It would depend on the day. When I went to school, i'd have 4/5 cups, but when I wasn't at school I'd have a lot more - the most that I've had in one day is 16 cups. I stopped drinking the coffee and nothing happened wth regards to my energy. I did become a lot hungrier though (it acted as an appetite supressant).
Now that I have M.E, I've been told that I'm not supposed to consume caffeine; although I haven't noticed a difference, so I drink decaf coffee. Tbh, I can't taste the difference, and I only drank it for the taste so it doesn't bother me. On the odd occasion I will have a caffeinated coffee (if for example someone doesn't have decaf coffee to give me), and the only effect it has is a laxitive effect if it's on an empty stomach, so I just make sure that I eat something with it . Likewise with sugar, I'm not suppsoed to eat/drink things that are high in sugar because of the 'crash' that follows, but that never happens to me either.
Tbh, I have noticed that a lot of things don't seem to affect me :S. Unfortunately, in my past I tried a lot of illeal things (I won't name them as I'm not sure if I'm allowed to), but they didn't have the desired effect. No matter how much I had, I've never been stoned, nor had all the other 'normal' effects :S. I have however been drunk (it was horrible and I don't desire to feel that way again - I only very rarely drink now, maybe a mulled wine at Christmas but that's it).