@Daphne Turnbull are you hitting it harder than normal?

@Daphne Turnbull You might have some fret wear going on there. I'd like to see a video of you moving power chords up and down the neck to see what's really going on.

I tried to duplicate on my guitar what you are experiencing and found that if I am not careful as to the placement of my index finger on that low E string, I can make the string buzz. In other words, make sure that you are not placing your finger on the fret itself. I did that, and sure enough, the string buzzed. If I adjusted the finger position, the buzz went away. As I moved up the fretboard, the power chords were easier to play and there was less of a stretch for my fingers and it was harder to make the string buzz.

Like Nate and Marian said, it could be fret wear or a technique issue. Even to this day I still sometimes have buzz when I'm playing power/bar chords on acoustic, always it's me slightly misplacing my fingers. Try this to check if your guitar needs an adjustment (or is worn). Play full bar chords (or power chords) all the way down the neck as far as you can. On every fret keep adjusting your fingers until it rings out clear without buzz, if it can be played cleanly then move to the next fret and play. If there is buzz even after you keep making careful adjustments then it's most likely an issue with your guitar setup (or worn frets).

I submitted a video for student review to get feedback on my power chord technique. I tried Nate and Chelsea's suggestions tonight, they helped and the chords sounded better.