How to turn off screen (DPMS) together with locking session in KDE?

Found a way for KDE 4 (up to Ubuntu 14.10). For modern KDE 5 see other answer.

1. Go to System Settings → Application and System Notifications → Manage Notifications.

2. Select Screen Saver as the Event Source

3. Locate and select the Screen Locked

4. add this command to the event:

    xset dpms force off
   

It will turn off all displays every time it gets locked.

Confirmed: running the command: /bin/sleep 1 ; /usr/bin/xset dpms force off within the screen locked notification works for Plasmashell 5.10.

Go to: System Settings > Notifications > Event source "Screen Saver":

With KDE Plasma 5.18, it is a little bit different from this comment.

Search for Notification via Super and follow the screenshots below:

I used /bin/sleep 5; /usr/bin/xset dpms force off instead of 1, but that's up to you.

As of KDE 5, most of the solutions here no longer work.

Here's what I found that works though (as of 2016-09-28), create a text document and in it copy and paste the following code:

#!/bin/sh
sleep 0.5
loginctl lock-session
xset dpms force off

and then save it as "KDE5lockscreenanddpms.sh" The name doesn't matter, just make sure you have the .sh at the end of it. Then navigate to where you saved with your file manager, right click on it and open up properties>permissions> and make it executable.

Now you can click on it and it will run the script which will lock your screen and then after .5 seconds will turn your monitor(s) off.

Here are some posts with the info about these commands.

KDE.org- loginctl lock-session

Linux-apps.com - sleep 0.5 xset dpms force off

One possible solution is switch to xscreensaver. The man page actually come with instruction.

Following is outline

1. Disable KDE screen saver

   1. K menu -> Computer -> System Settings -> Display and Monitor -> Screen Saver
   2. Uncheck Start automatically after

2. Install xscreensaver

   sudo apt-get install xscreensaver

3. Create ~/.kde/Autostart/xscreensaver.desktop

   Add following lines into it

   [Desktop Entry]
   Exec=xscreensaver
   Name=XScreenSaver
   Type=Application
   X-KDE-StartupNotify=false
   

4. Replace KDE lock screen

   cd /usr/lib/kde4/libexec
   sudo mv kscreenlocker kscreenlocker.original
   sudo vim kscreenlocker
   

   Add following lines into kscreenlocker

   #!/bin/sh
   xscreensaver-command -lock
   

5. Use xscreensaver-demo to configure dpms (no sudo)

   Configure DPMS in Advance tab. The minimum standby is 1min.

   However, you can try make it less by editing ~/.xscreensaver after first time configuration. Look for dpmsStandby: in the file.

   Not sure if it work if dpmsStandby: is changed to 0:00:00. I am testing with a VM and the black screen look the same.

You need a different solution, if you use a Wayland KDE session (running kwin_wayland).
As we - in the aformentioned scenario - are no longer using X.Org, X11 API and so xset - surprise - no longer works nowdays (as users are using less and less kwin_x11 based sessions).
But there should be some dbus method to turn off the screen(s), right? Right!

This solution is based on the accepted anwser and its KDE5 actualized copy.
So skim them - if you haven't done already - first. But the command needed is different now, and I have been inpired by a good topic on KDE screen turn-off shortcut.

Combining multiple info sources, bit dbus viewer trying and instrospect calls and tada - you should paste this command to the screen lock noticification:
/bin/sleep 1 && /bin/dbus-send --session --dest=org.kde.kglobalaccel /component/org_kde_powerdevil org.kde.kglobalaccel.Component.invokeShortcut string:'Turn Off Screen'

Some reasonin behind the command:

• The command starts with a 1 sec sleep delay because: the key unpress event of screen lock shortcut can immediately turn the screen back on
• The qdbus command is deprecated, and not installed on many distros now, but dbus-send is pretty basic and ubiquitous

Also, this should be moved to Super User, or Unix & Linux. Ubuntu is not the most relevant distro for KDE. but this applies to all KDE distros.

Kde5 and XScreenSaver(5.34) : click on tab complex Advance , then activate energy management by ticking it and set the minutes in power-off to your like (or use even quick power-off).

System Settings / Personal / Light Locker (in the new Whisker menu)

On Kubuntu 14.04, simply go System Setting - Power Mgmt. - turn off Dim Screen and Screen Energy Saving under Energy Saving tab.

Cheers!