I had a look at this questionthis question, and though about magic and the human body's energy output.
I remember calculating the energy require to walk up a 3.4m staircase quickly in a school physics lesson once. I looked up the results in my old book, and I got the power outputs of everyone in the old class. The greatest output was around 840W in 4 seconds, and the least was 130W in 14 seconds.
I remember being surprised by this, especially considering that a really good lightbulb is 17W, and an LED lamp is about 60W. It doesn't take too much energy to start a fire, either, though admittedly it probably takes much more to sustain one.
If 'magic' in humans were possible by using energy produced by the human body, what limits would this magic have? How would this magic differ from the Stereotypical, Tolkein or Harry-Potter style magic we're used to? What effect would this have on people using magic?
And, more importantly, what would magic be able to do that we can't do better with electricity?
In response to PipperChip: I don't see why it should be constrained to any type of energy, so molecular energy and such is viable in my eyes.