I wasn't really thinking much about it, tbh, but it's more along the lines of "cell burns glucose to create energy", or "random photon hits skin cell, making it slightly warmer," or in the more unfortunate circumstances where the random part of the body is, say, a brain cell, it could have not much choice but to "spontaneously undergo nuclear fission."
It's me trying to cover my ass, tbh, and make things more interesting by just not going "E=MC² thus things go boom!"
I suppose you can go use the power safely most of the time, but there's a chance that something important might go off in the most unfortunate way.