I use KDE 5.15.3 on the latest KDE Neon, and recall this problem happened with Linux Mint 19 + an older version of KDE (don't recall which) as well.
Intermittently, after booting I don't have the battery-widget in my task-bar (so I can't see how much battery is remaining). When this happens and I try to open the energy saving dialog, it tells me
Power management configuration module could not be loaded.
The Power Management Service appears not to be running.
This can be solved by starting or scheduling it inside "Startup and Shutdown"
Also, when this occurs, Hibernate/Standby option is missing from the shutdown menu.
Usually the problem clears when I reboot the system; sometimes log out and back in clears it. Either resolution is not permanent.
Note: When I go to System settings > Startup and Shutdown I don't have a tab called Service manager (as suggested in many threads I found on this topic). I only have Background Services and when choosing this one, there is no entry containing the word Power (Neither is it in any other tabs I see under Startup and Shutdown). And I don't have an option to add anything to the list.
Running journalctl -r contains the following, which seems kinda related to the problem:
Apr 01 12:35:20 workstation dbus-daemon[785]: [system] Reloaded configuration
Apr 01 12:35:20 workstation dbus-daemon[785]: Unknown group "power" in message bus configuration file
Apr 01 12:35:20 workstation dbus-daemon[785]: Unknown username "whoopsie" in message bus configuration file
Another solution suggested here is to re-install upower, but this didn't have any effect on the situation. Additionally ps -e | grep upower does find 5399 ? 00:00:00 upowerd so I guess the problem is not, that upowerisn't running.
EDIT:
Running journalctl -b 0 | grep -i 'powerdevil' as suggested in the comments, yields this output when the problem occurs:
Jun 30 09:38:35 workstation dbus-daemon[751]: [system] Activating service name='org.kde.powerdevil.discretegpuhelper' requested by ':1.37' (uid=1000 pid=1271 comm="/usr/lib/x86_64-linux-gnu/libexec/org_kde_powerdev" label="unconfined") (using servicehelper)
Jun 30 09:38:35 workstation org.kde.powerdevil.discretegpuhelper[1347]: QDBusArgument: read from a write-only object
Jun 30 09:38:35 workstation org.kde.powerdevil.discretegpuhelper[1347]: QDBusArgument: read from a write-only object
Jun 30 09:38:35 workstation org.kde.powerdevil.discretegpuhelper[1347]: QDBusArgument: read from a write-only object
Jun 30 09:38:35 workstation dbus-daemon[751]: [system] Successfully activated service 'org.kde.powerdevil.discretegpuhelper'
Jun 30 09:38:35 workstation dbus-daemon[751]: [system] Activating service name='org.kde.powerdevil.backlighthelper' requested by ':1.37' (uid=1000 pid=1271 comm="/usr/lib/x86_64-linux-gnu/libexec/org_kde_powerdev" label="unconfined") (using servicehelper)
Jun 30 09:38:35 workstation org.kde.powerdevil.backlighthelper[1358]: QDBusArgument: read from a write-only object
Jun 30 09:38:35 workstation org.kde.powerdevil.backlighthelper[1358]: QDBusArgument: read from a write-only object
Jun 30 09:38:35 workstation org.kde.powerdevil.backlighthelper[1358]: QDBusArgument: read from a write-only object
Jun 30 09:38:35 workstation dbus-daemon[751]: [system] Successfully activated service 'org.kde.powerdevil.backlighthelper'
EDIT2:
After doing some more research on this topic I stumbled upon these GitHub issues:
- https://github.com/NixOS/nixpkgs/issues/47760
- https://github.com/NixOS/nixpkgs/issues/24563
I did inspect my ~/.config/ksmserverrc but I don't seem to have weird paths in there. Also I do start with an empty session anyways so this shouldn't be a problem anyways...
journalctl -b 0 | grep -i 'powerdevil'? You may want to exclude messages with low priority with, e.g.journalctl -p 0..7and search messages about previous boots with e.g.journalctl -b -1(last boot; -2 is second to last, ...).