In physics, energy is simply the ability to do work. Objects can have energy by virtue of their motion (kinetic energy), by virtue of the position (potential energy), or by virtue of their mass (see E=mc2). None of this can be said of qi, prana, or any other alleged 'vital life force.' When someone talks about the "energy of a group of people" and they are speaking metaphorically we can -- for the most part -- understand what they are saying when they say "the energy of this group is very strong."
The confusion arises when new-agers talk about "non-material energy" and invariably refer to Einstein's famous equation: E=mc2 using it incorrectly to assert it is saying that material mass can be turned into nonmaterial energy (and vice versa). In fact, the equation is stating that energy is a quantifiable property of a material object. That is to say, a material object doesn't turn into energy, but into other material objects that carry energy.
Einstein's equation is the 'rest energy' of an object that has mass. It is stating the possibility of extracting E amount of energy from m kilogram of mass. One kilogram of uranium sitting in a stock pile has no energy, but if it is lifted several feet off the ground, it now has some potential energy and once it's made to enter into nuclear interactions, it's mass turns into energy carried away by the nuclear particles produced by the interaction. There is no place for woo. So, when I say, upon entering a room of new yoga students, that the energy seems "strong" or "high" I am simply saying that there seems to be a general excitement shared by the group that I can sense.
However, when new-agers and spiritualists use the term "energy" as in reference to one's body's "energy field," they're really saying nothing even remotely meaningful. Yet, as Brian Dunning has written, "this kind of talk has become so pervasive in our society that the vast majority of Americans accept that energy exists as a self-contained force, floating around in glowing clouds, and can be commanded by spiritualist adepts to do just about anything." For instance, alleged Qi or Chi Masters are said to be able to move objects without touching them. A whole school of martial arts is supposed to be based upon the control of qi where a master can take down a slew of opponents without touching them. Of course, if you have complicit students willing to believe anything can seem possible then near miraculous effects can be simulated, but when such woo fantasy meets reality, reality wins. Ironically, the MMA martial artists who exposed the delusion of this qi martial arts "master" was excoriated by many in China for insulting tradition!
Dunning suggests that "when you hear the word "energy" casually used to explain a mystical force or capability, require some clarification. Require that the energy be defined. Is it heat? Is it a spinning flywheel?
He offers this good test: "When you hear the word energy used in a spiritual or paranormal sense, substitute the phrase 'measurable work capability.' Does the usage still make sense? Are you actually being given any information that supports the claim being made? Remember, energy itself is not the thing being measured: energy is the measurement of work performed or of potential."
Dunning gives a good example from a claim made by Kundalini Yoga adepts:
The release and ascent of the dormant spiritual energy enables the aspirant to transcend the effects of the elements and achieve consciousness.
He writes: "This would be a great thing if energy was indeed that shimmering cloud that can go wherever it's needed and perform miracles. But it's not, so in this case, we substitute the phrase "measurable work capability" and find that the sentence is not attempting to measure or quantify anything other than the word energy itself. We have a "dormant spiritual measurable work capability" and no further information. That's pretty vague, isn't it? For this claim to have any merit, they must at least describe how this energy is being stored or manifested. Is it potential energy stored in the chemistry of fat cells? Is it heat that can spread through the body? Is it a measurable amount of electromagnetism, and if so, where's the magnet? In any event, it must be measurable and precisely quantifiable, or it can't be called "energy" by definition."
Dunning continues: "There's a good reason why you don't hear medical doctors or pharmacists talking about energy fields: it's meaningless. I think it's generally good policy to remain open minded and be ready to hear claims that involve energy, but approach them skeptically, and scientifically. The next time you hear such a claim, substitute the phrase "measurable work capability" and you'll be well equipped to separate the silly from the solid."