rc.local is not the good way to do thing and may not work at all. Use the recommendations from the Power saving page on the Arch Wiki instead.
So how do I use it ? Well, I found the html report to be the most useful as it gives you the exact commands (Tunables) you need to run to adjust power settings:
You will then find those commands in the
Tunables tab in the
powertop-*.html generated file. You should try carefully each one of them one by one as they can have some nasty effect (disabling USB mouse, increasing power consumption…).
If you’re using
systemd like I do, and want to use the classic
/etc/rc.local file to set those settings at boot time, you should use the
initscripts-systemd package and make sure it is enabled with :