Lately there has been an increased interest on worldbuilding.se about a realistic depiction of fictional creatures who would feed off thoughts and dreams, with some answers proposing feeding off the electromagnetic emissions of the brain.
As a first guess I would think the energy in those emissions is very small. However, has there been any study to give an approximate amount? Most information a quick search provides stems from sensationalist articles which don't look too trustworthy.
The brain does use up almost 20% of the whole body's energy balance, but I would guess most of it is turned into heat.
If one could extract useful work out of the electromagnetic field generated by an average human brain with 100% efficiency, without any physical intrusion (so, the energy potentially available outside of our head), how much would it approximately be? An order of magnitude approximation would be enough.