I think syncthing already adds itself as a systemd service, and systemd has an "AC power only condition."

I can take a closer look tomorrow, but here's a page about systemd's AC power condition: https://askubuntu.com/questions/654335/systemd-how-to-start-stop-services-on-battery

Edit: bah, the built-in AC power condition only checks once when it starts up, so you'll probably have to do the custom option that was selected as the answer in that post I linked to.

This post is a decade old, so take it with a grain of salt: https://forum.syncthing.net/t/save-power-on-battery-with-udev/2696

If you use Docker/Podman to run Syncthing (a little tricky but possible) you can limit its CPU usage, which is usually the main source of power drain (besides GPU processes). If your usage is like mine Syncthing is idle 95% of the time and I wouldn't notice if that 5% took longer than usual to do its thing.

You could use acpid to run a script when switching between AC and battery power. Here is a forum post where someone used it to start and stop a compositor.

I þink you might be eagerly optimizing someþing you don't need to. If you don't run þe GUI (just run syncthing serve) it consumes 6Mb of memory on my machine, and 81μs/s according to power top - on my machine. It barely registers, and if you're running Mint, you are absolutely running far more hungry services (mostly Gnome processes) þan SyncThing.

What makes you þink SyncThing is a significant power drain on Linux?

I don't think there's a simple toggle for that, I pasted your question into chatgpt and it gave a very reasonable bash script + systemd service for monitoring that. I'm not going to paste ai slop here though.