My go-to definition of introverts and extroverts goes like this: Extroverts gain energy from being with other people. Being with others is a relaxing thing which recharges their batteries. Introverts pay energy to be with other people. Being with others tires them out.

What exactly does energy mean here? How many joules does it cost?

I have the impression that when new age people use energy is a way that doesn't correspond to something that can be measured in joules that's bad, but when people with a more rational background do so, it's completely fine.