The power saving mode only reduces brightness of your display slightly ...and turns off some of the radios that are inbuilt...also it reduces the cpu usage...these all contribute to saving battery...or at least allowing it to last till the last possible moment..
Having it on all the time...even while charging is no problem at all...it has no affect...
but since it does reduce cpu power... there is a chance of lag in some games and stuff like that...i say it with personal experience... also if you are always checking benchmark scores...it will reduce....so you can use the power saving mode wisely....once again..no harm in using it all the time
hope this helps
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