Once you activate it, Granola reduces energy consumption between 10% and 35%, even when you're playing demanding games or have several different applications open at the same time. Apart from that, it saves energy and shows you how much you're reducing the advance of the greenhouse effect.
Usually Granola starts working right after you install it, but if your computer is already a few years old, you might have to make slight changes to the BIOS. Either way it's not very hard, and there's a tutorial on how to do it.
Granola doesn't negatively affect your computer's performance, and it helps reduce energy consumption. It seems like a great application to have no matter how you look at it.