There's a sensation of having a high muscle tonus. There a sensation of feeling the need for sleep. There's a sensation that comes with an increase in testosterone. There's a sensation of reduced inhibition for movement. There's a sensation of motivation. There's low/high blood sugar. There's heart rate variance. There's willpower.

Those are all different variables that person could mean when he speaks about "paying energy".

If you are interested in actually understanding what happens when introverts pay energy for social interaction it's useful to mentally distinguish those things. Distinguishing them allows you to intervene.

I personally don't drain when in social interaction. But I'm also not a typical extrovert.