https://support.apple.com/en-mn/guide/mac-help/mh40774/mac
This seems to be available as a first party config option
Edit 2: My bad, I assumed power nap == "wake for network access". This no longer seems to be an option in macOS 26.
Mine's set to "Only on Power Adapter", which makes sense.
Edit: On an M4 MacBook Air running macOS 26 DB
On macOS 26 DB running on M4 MacBook Air, I don't see power nap in that place. I see "Wake for network access" which might be the new thing and it's set to "Only on Power Adapter" by default.