A simple script to track the battery discharge rate during suspend.
Copy the script to /usr/lib/systemd/system-sleep/:
sudo cp powerdrain /usr/lib/systemd/system-sleep/
The discharge rate will be logged after each suspend/resume. Nothing will be logged if system is suspended with connected PSU.
% journalctl -u systemd-suspend.service | tail
Sep 11 12:54:33 laptop systemd-sleep[19801]: Suspending system...
Sep 11 12:55:37 laptop systemd-sleep[19801]: System resumed.
Sep 11 12:55:37 laptop systemd-sleep[19841]: [INFO] Duration: 00:01:04
Sep 11 12:55:37 laptop systemd-sleep[19841]: [INFO] Discharge rate: 1.12549 W/h.