This is a VST (Very small tip) :).

How to run xterm with white letters on a black background, opposite to the default black on white.

This should (IMHO) increase the duration of your laptop battery as less bright is used.

xterm -ls -fg white -bg black

You can create an alias for this command, like this:

alias xtermblack="xterm -ls -fg white -bg black"

And add that line to your .bashrc file, so it can be set each time you boot up your computer.