There's nothing within the operating system that can do this. As opposed to
powering down the system, you could let the system go into standby,
something they system can be set to do after a period of inactivity. That
can period of inactivity can be set as follows. Open Control Panel, open
Display, go to the Screen Saver tab, click the Power button under Monitor
Power, choose a setting from the dropdown list for System Standby.
If whish some added security, assuming you require a password to log on, at
the Screen Saver tab, you can set the system to display the Welcome screen
on resume and this would require you to or anyone who simply hit the
spacebar to enter a password to return to the desktop but that's up to you.
Also note, there is a setting for turning off the monitor after a period of
inactivity as well and you should make such a setting in order to save the
monitor during this period of standby assuming you choose to use this
option.
If you don't wish to leave the system running 23/7, though in a low power
mode, this would require a third party option, most likely some hardware
solution.
