To me, a waste of energy is usually an activity or pursuit that consumes time, brainpower, and other intangible personal resources, but doesn't really have a point or yield useful results. I'm thinking of tasks like mindless busy work you might be given at a place of employment or things your parents gave you to do when you were a kid just to keep you out of their hair. They're wastes because that same energy could just as well be spent on something that feels worthwhile because it's actually fun, useful, or productive.
Sometimes I think of other scenarios as wastes of energy on a larger scale though. Examples from my life would include some of my past relationships. I used to have a bad habit of getting into relationships with people I wasn't all that into just so I wouldn't have to be single. In some of those cases, I stayed with the person for years despite being 100% aware that I didn't see any sort of a future with them. Even casual relationships consume a lot of energy though, so I look back on situations like that as having been absolute wastes. I could have been spending that energy on bettering my own life or on people that were better suited to me. I also think back on some of my old jobs.
When you combine all of those situations and look at them as a collective, they add up to entire years of my life flushed straight down the toilet because they yielded absolutely nothing of use. That time and energy is something I'll never get back. I wonder all the time how my life might be different or more stable if I'd actually spent those years on worthwhile people and pursuits instead. Not every life experience has to be earth-shattering or life-changing, but it would be nice if I could at least look back on some of those situations as having been fun for what they were at the time.