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,
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.
journalctl -r contains the following, which seems kinda related to the problem:
Apr 01 12:35:20 workstation dbus-daemon: [system] Reloaded configuration Apr 01 12:35:20 workstation dbus-daemon: Unknown group "power" in message bus configuration file Apr 01 12:35:20 workstation dbus-daemon: 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