XFCE - How To use both Screensaver and Power Management
It is difficult to make power management of the monitor work when using an XFCE screensaver. There are actually two different places where one can make power management settings for the monitor.
The Power Manager icon on the panel is the most obvious place to set the monitor power management. Unfortunately those settings don't work with the screensaver. If the screensaver starts then the monitor never suspends.
When configuring the screensaver there is an "Advanced" tab containing settings for "Display Power Management". Those settings do work properly and the "Suspend After" parameter shuts of power to the monitor.
To configure a screensaver and monitor suspend do the following.
Right click on the Power Manager icon on the panel and then click "Preferences". In the Power Manager Preferences click on the monitor icon next to "Monitor Settings". For the "On AC power" tab enter 0 for all three settings. They should say "0 never" when you are done. Click the "On battery power tab". Enter 0 for all three of those settings. Click the "Close" button.
Click the mouse icon on the panel to open the XFCE menu. Click "Settings" in the menu and then click "Screensaver". In the Screensaver Preferences, configure the desired screen saver on the "Display Modes" tab. The setting for "Blank after" controls how long a delay there is before the screensaver activates (when there is no mouse/keyboard input). You can enable the "Lock Screen After" setting and then enter how many minutes to wait before locking the session (require password) following activation of the screensaver. The "Cycle After" setting is only important if you choose random screensavers. It controls how long each screen saver is displayed before going to the next one.
Click the "Advanced" tab. Put a check mark in the box next to "Power Management Enabled". Click the box if it doesn't already have a check mark. Enter 0 for the "Standby After" setting. Enter the desired number of minutes for "Suspend After". Note that the time is concurrent with the screensaver time. If the screensaver is set to start after 15 minutes then setting "Suspend After" to 30 will cause the display to shut off 15 minutes following activation of the screensaver. If you set "Suspend After" shorter than the screen saver delay then the screensaver will never start (and the session will also never be locked). Enter 0 for the "Off After" setting. Your monitor or display may support other power features so you may be able to use the other settings. So far "Suspend After" seems to work on all the monitors and laptops that I've tried.
The only way that I've found to lock the XFCE session is by enabling some screensaver (even if it's just a blank screensaver). The screensaver has to actually start in order to lock the session. If the computer is or monitor is put into standby, suspend or power down before the screensaver activates then the session will not be locked when the computer (or monitor display) resumes.
I have not experimented with the other settings in the Power Manager or Screensaver. There may be other Power Manager settings beside those for the "Monitor" that affect operation of the screensaver. There also may be other ways to lock the session.