For over 25 years I wondered why social events drained me and what neurological phenomenon the metaphor "energy" stood for (which this article and many others like it don't seem to bother investigating). I've recently come to realize that it's the extent to which you see other human beings as a threat, which causes your brain to keep its "shields up", which can feel exhausting. After a observing myself a lot during social interactions, I've noticed that I feel drained to the extent I think the people around me can potentially judge me: I feel highly drained during extended family events, class reunions or ceremonies, but I feel perfectly fine amongst a small group of friends who have a don't-give a-sh*t attitude toward social propriety. I've also noticed that alcohol (a disinhibitor) can dramatically reduce the drain. It's also significantly reduced if I find a table with a few people I like, and get engrossed in a deep discussion and end up forgetting that I'm surrounded by other tables.
Perhaps this energy we speak of is the energy the brain expends to continuously compute the requirements of the social contract in members of our species who don't have it sufficiently hardwired.