There's a solution for this problem here https://bbs.archlinux.org/viewtopic.php?id=195300
. That's what kurotsugi meant with "backend". If the GUI doesn't have it just edit the config file manually - remember you need sudo permissions for this and backup before editing. I haven't tested the solution, I'm not on laptop atm.
There also seems to be a bug involved https://bugzilla.xfce.org/show_bug.cgi?id=12756
but it's probably only the GUI. But if you don't get it to work by editing the config you could try mate-power-manager. It's in the repo.
Offtopic: Are you the same person as melp57 just posting with different user names? Please decide on one if at all possible.