Conky is a really great system monitor application for X, however, since it does not work out of the box, one has to write a configuration to set up what to monitor and how to display it. Although there are tons of themes online (e.g. opendesktop.org or DeviantArt.com) I still decided to post my quite old (~10yrs), streamlined, but still informational configuration for laptops. It will fit any screen that has at least 768px vertical resolution.
Clean laptop conky theme running on Plasma
REMAINING TIME line shows
AC ON-LINE when connected to a power outlet
and fully charged, or the time remaining to full charge if battery is charging.
Otherwise the estimated usage time remaining is displayed while running on
battery. It also has a separate
TMP storage space indicator as that’s mounted
$ conky -c ~/.conky/conkyrc_transparent &
You might want to add it to your window manager’s autostart scripts to launch it on boot.