The sun.
Almost all the available energy on Earth originally came from the Sun; the only other sources I know of are radioactive elements within the Earth and the rotation of the Earth-Moon system.
So even if it's not from the sun's current output, it's probably going to be from the sun's past output.
r/Fitness does a weekly "Moronic Monday", a judgment-free thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. I thought this seemed like a useful thing to have here - after all, the concepts discussed on LessWrong are probably at least a little harder to grasp than those of weightlifting. Plus, I have a few stupid questions of my own, so it doesn't seem unreasonable that other people might as well.