Serious question. Don't say "it's the ability to do work," because we both know that work is "change in energy."

In freshman physics, you've got your forces and accelerations, but everything interesting is a mess of differential equations you can't solve. Then someone comes along and says, "here's a neat trick: just calculate the energy like this and forget about everything else," and that turns out to be a great way to look at things. But when I got to

BabyQM, there's Schrode's equation with its energy term, and suddenly I realized that I have no idea what energy is. It was always just a trick for solving mechanics problems. So what's going on with this energy term? You could apply a voltage between a couple of plates and shoot your electron in between there. But that's just electron-electron repulsion, so it seems like E is just some kind of statistical average of how likely your electron is to interact with other nearby electrons. It's classical thermodynamics all over again. Or am I way off base here?

So anyway, does anyone have a snappy non-self-referential answer to the question "what is energy"?