After spending a full afternoon at a dinner party with colleagues, do you feel a) happy, refreshed, and ready to do it again tomorrow or b) drained, tired and wanting to just be alone for a while to recuperate? I'm the latter, which is traditionally classified as introverted. To flip it around, the extrovert would feel drained, tired, and wanting of some social interaction after spending an afternoon sitting in front of a computer (my wife is like this).
Energy is a metaphor for the "stress level," of the individual for lack of a better term.
I'm the same and did not discover this until about the 5th time I took the Myers-Briggs Personality Test. My initial tests had me almost exactly between extroversion and introversion.
On my last test, the facilitator explained the energy/stress aspect with a number of examples like your dinner party. She also mentioned people are naturally one or the other, not 50/50.
I realized that my natural inclination was introversion. I believe that what I perceived as my extroverted side was learned through a social upbringing to engage others, which is beneficial. There are a number of social/extroverted things that I sincerely enjoy but I walk away exhausted.
Energy is a metaphor for the "stress level," of the individual for lack of a better term.