Don't load qml, register model if launch in daemon mode. I didn't test the cpu usage difference, but the memory usage is significantly reduced in daemon mode. (67 MiB -> 5.4 MiB)
This would be useful when kweather plasmoid is added to desktop and user doesn't open kweather window.
The more aggressive power saving method would be destroying QQmlApplicationEngine instance when last window closed and re-construct it when DBus calls to activate KWeather GUI. However, there is overhead in destroy and re-creation GUI and the trade off will need to be discussed.
This is somewhat related to general power management in Plasma Mobile, any input from Plasma team is well appreciated.