Energy in music is a funny thing. It's easy to hear, but it's tough to define or quantify. People try to fake or manufacture it all the time -- making music that's faster or more chaotic than it might otherwise be -- but music made like that can often sound as rote as anything else. Much of the time, energy is tied to formal innovation of some sort; you can hear borders being pushed in one way or another, and that adds to some hazy feeling of excitement. But music with energy doesn't have to make advances. Plenty of knowingly, consciously retro music -- the Stray Cats, the early White Stripes, Missy Elliot's Under Construction -- can have more energy than almost anything else. It's entirely in the ear of the beholder. You can't really describe what makes one piece of music more energetic than another. But ideally, you know it when you hear it.