Mine's set to "Only on Power Adapter", which makes sense.
Edit: On an M4 MacBook Air running macOS 26 DB
https://support.apple.com/en-mn/guide/mac-help/mh40774/15.0/...
Turn Power Nap on or off for a Mac desktop computer On your Mac, choose Apple menu > System Settings, then click Energy in the sidebar. (You may need to scroll down.) Turn on Enable Power Nap. Note: This option is only available on Intel-based Mac computers.
You can see (and change) the settings via Terminal, 'pmset -g' will show the current options.
> sudo pmset -a powernap 1
-a is an option to set it for battery and plugged-in. If you want only either you can do -b for battery, -c for charger
You can also check the settings with:
> pmset -g