In last few releases linux since 15.0 I've been unable to set my to computer to low-power as it does not awake correctly. It keeps displaying a black screen, that cannot be anyway. It may be related to hardware low-power problems in any device (p.e. USB hub).

Nevertheless, these computers with the black screen are running 'fine' and can be gracefully turned-off by pressing their 'power' button. One rebooted everything is back again.

I finished by cancelling any lowpower statuts setting, undesirable, tryig to avoid this annoying bug. Sadly, i worked this way for 2 years. My tested computer is a Toshiba Portege R200 with Intel i915 graphics.

I searchec the web several times before finding the explained solution (see references)

The solution is very simple: Let Powertop to tune your system (solved).

a) Add Powertop to your computer

sudo apt-get get install powertop

b) Run a non-destructive test.

Run powertop.

sudo powertop --auto-tune    (it fixed all but one of my 18 errors on the fly. It just worked !)

     (You may investige your error details by running sudo powertop ,and then navigate with [tab] or [esc])

Now, test a lowpower and awake/power-up cycle. It should work from the the shutdown menu or from the timed energy/power control panel.

It works fine ? Good. Youy may consider to add powertop to your system booting process

c) Set powertop permanent,  changes will be in a editable file (it is a safe way to keep them)

Add your previously successful command line to /etc/rc.local  (edit it; p.e. sudo nano /etc/rc.local)

Add the command just before the last line before exit 0

# By default this script does nothing.

powertop --auto-tune
exit 0

Now powertop will fix the lowpower problem when you start your system.

References:

1. Powertop info and  Permanent Changes : http://askubuntu.com/questions/112705/how-do-i-make-powertop-changes-permanent
2. Powertop good bad recommendations/ http://kernel.ubuntu.com/~cking/power-benchmarking/powertop-good-bad-recommendations/
3. Unsolved thread to avoid...  ubuntuforums.org/showthread.php?t=2180234