I noticed my Dell Latitude laptop running Kubuntu 16.04 that even after shutting down completely, the battery was still being drained. The problem is that wake-on-lan is generally turned on by default. Unless you need WOL for some reason — turn it off.
The core method is to create a file named wol.service in /etc/systemd/system/ with:
Then run: sudo systemctl enable wol.service
Run sudo ethtool