Saving Power on idle PCs

General Introduction
Saving Power at the CPU
  CPU Introduction
  AMD processors
    Athlon XP and MP, Duron
    Sempron, Athlon XP Mobile
  Intel processors
    Pentium II
    Pentium III
    Pentium4 Mobile
    Core/Core Duo
    G3 and G4 desktop
    G5 desktop, G3,G4, and G5 laptops
    iMac Intel Core processor
Saving Power at the Display
  Display Introduction
  Windows Display Control
  Macintosh Display Control
  Linux Display Control
Technical info and raw data
  Windows XP power definitions
  Windows XP power settings
  Power consumption measured on various processors
  Power consumption measured on various displays
  Power consumption measured on various screen savers
  Power consumption measured on various printers
  Calculating power costs
  Replace CRTs with LCDs to save money?
  Blanking screen vs. turning off displays
Links to software

General Introduction

This page describes methods for saving power on computers using various CPUs when the machines are idle. For this discussion a machine is idle when it is in a normal operating state and nothing is running on the CPU. For instance, on Windows XP it corresponds to the amount of time spent in the "System Idle Process". When a computer is used for word processing or similar data entry tasks, or displaying static documents, it will spend the vast majority of its time idling. However, if music is simultaneously playing in the background, or a USB device is active, the same machine may spend almost no time idling.

Computer servers may also save considerable energy by appropriately configuring their display options. Since typically the consoles of such systems are rarely used the display and parts of the video circuitry may be powered down without penalty. If the display is needed pressing a key on the keyboard or shaking the mouse is sufficient to bring the display back on line.

Most operating systems can also be set to drop the machine into a standby state following a long period of inactivity, or on a notebook, when the system is folded closed. Typically this is much more drastic than just slowing the CPU and turning off the monitor. For instance, the disks are spun down and the system effectively shuts itself off. These states are not further considered here because they represent systems which can do no work. They are not merely idle (either at the CPU or the Display) but for most purposes, they are simply "off".

Saving Power at the CPU

CPU Introduction

Manufacturers have designed certain features into their processors that enable said processors to drop into lower power states when signaled by the operating system that the machine is idle. The primary mechanisms are:

  1.  Decreasing the CPU frequency
  2.  Decreasing the CPU voltage
  3.  Temporarily disconnecting the CPU and reducing it's frequency.

By installing software to activate these capabilities the power dissipated by the idling CPU may be substantially reduced. For instance, on one Athlon XP 2000+ machine the CPU dissipated 39 fewer watts, which resulted ina reduction of the CPU temperature from 59° C to 47° C.

Additionally many CPUs may be throttled, which effectively forces the processor to be inactive during some part of each second. Throttling is typically used to control overheating on heavily used machines. However it generally doesn't save more power when the machine is already idle, and it tends to adversely affect computer performance to such an extent that the machine feels "broken", so throttling will not be further discussed here.

AMD Processors

Athlon XP and MP, Duron

These CPUs can respond to an idle state by disconnecting from the front side bus and dropping their CPU frequency. In order to do so two PCR registers in the northbridge must generally be modified. In addition the length of time for a Halt or StopGrant disconnect may be controlled by a clock divisor set in the MSR. In experiments with Athlon XP 2000+ machines this divisor could be set as high as 128 without causing problems, but going to 256 caused various glitches in video and sound playback. The idle temperature decreased by about 2° C for every doubling of this divisor. Your mileage may vary.

Windows XP Instructions

  1.  Login as administrator
  2.  Obtain coolon 2.1.0 from
  3.  Create a folder C:\Program Files\Coolon
  4.  Run the coolon installer and unpack the files into
      C:\Program Files\Coolon
  5.  Double click C:\Program Files\Coolon\coolon.exe
  6.  Click on the PCR tab and check cooling
  7.  Click on the MSR tab, click Change
      Change the Halt and StopGrant values to 128
        WARNING!  Your system might require smaller values!
      Click on eVal
      Click on Write
  8.  Click on Shortcut
      Select Startup Tab
      Check MSR and Cooling
      Click on Make
  9.  Click on Shortcut
      Select Desktop Tab
      Click Make for MSR, Cooling, Cooling On, and Cooling Off
  10. Click on Driver tab
      Check Startup Service
      Click on Set
  11. Close Coolon
  12. Right click on:
        C:\Documents and Settings\All users\Startup\Programs\Startup\Coolon
      Select properties
      Change the options on the startup string to:  /M /C
      (eliminate the /D, it will cause problems for unpriv'd users.)
  13. For both the Coolon On and Coolon Off desktop icons.
      Right Click on the icon.
      Select Security tab.
      Add or modify so that it has: Everyone: read & execute, read
      (This lets an unpriv'd user turn cooling on or off.)
  14. Change the file protections on the Coolon directory and its
      contents to: Everyone: read & execute, read, list folder contents
  15. Monitor the CPU temperature with the system at idle.

Note added 1/19/10.  As of this date fully patched XP SP3 does not enable
power regulation of this processor by itself.  So the steps above are still

Linux Instructions

See the athcool home page if you require more information.

  1.  Use a recent 2.6.x kernel.
  2.  Boot with kernel option: acpi=strict
  3.  Login as root
  4.  Obtain and install the athcool package (as appropriate
      for your distribution).  If no package is available build
      it from the tarball on the athcool home page.
  5.  See if athcool works:  athcool stat
  6.  Start athcool:  athcool on
  7.  If the CPU temperature falls (sensors command, from
      lm_sensors package) and nothing blows up, then enable
      athcool to autostart at boot.  Some athcool packages provide
      an init.d script for this, others don't.
  8.  If athcool doesn't work or the CPU temperature doesn't fall on
      the idle machine (preferably with no X11 running, no USB devices,
      top showing little or no CPU use) try a more recent kernel
      as the relevant parts of the kernel code may have been fixed.
  9.  Still not working?  Do:  modprobe processor
  10. cat /proc/acpi/processor/CPU0/info
      and see if acpi is disabled in the kernel.
Note added 2/28/07.  Some distributions such as Mandriva 2007 do all of
this automatically.  In addition to athcool they use a powernow-k7 module.
You can verify that athcool is working by using
lm_sensors to measure CPU temperature on the idle system, turn off athcool,
wait 20 minutes, and look at the CPU temperature again.  Also, athcool did
not work on a Tyan S2466N mobo with an Athlon MP processor under early 
2.6.x kernels, but upgrading those machines to Mandriva 2007 let athcool
work correctly, dropping the average CPU temp from 39C to 27C.  

Note added 06/06/07, modified 6/15/09.  Athcool sets two bits and just leaves them on.  So
this saves power when idle but does in some cases affect the machine's
performance while busy.  For instance, on our S2466N machines the IDE
read/write performance decreased by up to 50%, and changing the PCI latency
values did not help.  To work around this I wrote a small perl script
that toggles athcool off if the system is busy, and on if it is idle.  This
is similar to what cpufreq does on new AMD processors, where the frequency
and voltage are adjusted up or down as needed.  The script
should be started by the init.d script ctrl_athcool.  Disable
the init script for athcool, it is not needed once the ctrl_athcool init script is installed.
The Perl script uses athcool to control the power saving bits, but it also
monitors CPU and network usage, and turns the power savings back off again when
the system is busy.  The current version of the Perl script daemonizes and the init
script works with RedHat derivatives like Mandriva.

Note added 02/08/08.  Found that Athcool negatively effects heavy network load
even when that load does not cause much of an increase in the CPU load.  Modified to shut athcool off after it detects a significant network load.
The trigger is the sum of the number of packets read and written within a 1 second
interval.  Either network load or high CPU usage now causes athcool to turn off, both
conditions must abate before athcool will be turned back on.

Sempron, Athlon XP Mobile

These processors support Cool'N Quiet, which is a reduction in both clock frequency and voltage at idle. In general they use less power than the regular Athlon XP and MP processors. Tests with Central Brain Identifier indicate that on the experimental machine the Halt instruction is enabled and accounts for about 18 Watts of power saving. Stop Grant was disabled, but enabling it made no difference, possibly because CBID did not work properly with the DFI motherboard. In any case, no manual intervention was required to enable Halt.

Windows XP Instructions

   1.  As administrator
   2.  Go to and find the relevant download page for
       the processor in question.
   3.  Download the processor driver
   4.  Download the AMD power monitor
   5.  Install both.
   6.  Run the power monitor and set it to: minimal power management.
   7.  The power monitor should show that when the machine is idle the
       frequency drops by about half and the voltage drops by about 30 percent.
   8.  You do not need to leave the power monitor running in the foreground.
   9.  When a normal user logs in subsequently the power monitor may show
       home office and not minimal power management.  Yet it
       appeared to work as if it was in the latter mode.  To be safe have
       normal users do:  start -> control panel -> power savings
       and set it to minimal power management
   10. Note, it isn't necessary to use the AMD power monitor.  Setting
       minimal power management in the power options control
       panel is equivalent.  However the power monitor shows that the expected
       changes have actually transpired.

Linux Instructions

Not written yet. (Try the powernowd tool.)


Windows XP Instructions

   1.  As administrator
   2.  Go to and find the relevant download page for
       the processor in question.
   3.  Download and install the processor driver
   4.  Set minimal power management in the power options control

Linux Instructions

   1.  Use a recent 2.6 series kernel.
   2.  Verify that the module powernow-k8 exists.
   3.  Verify that powernow-k8 will load with: modprobe powernow-k8
         (note, if your BIOS does not support the PowerNow for this particular
         processor you will something like this:
       FATAL: Error inserting powernow-k8 (/lib/modules/ No such device
         and % dmesg | tail  will show (on a dual CPU system)
       powernow-k8: Found 2 AMD Athlon 64 / Opteron processors (version 1.60.2)
       powernow-k8: MP systems not supported by PSB BIOS structure
       powernow-k8: MP systems not supported by PSB BIOS structure
   4.  Install cpufreq and cpufrequtils
   5.  The defaults are probably ok. Use cpufreq-info and cpufreq-set to change
       things, or modify  /sys/devices/system/cpu/cpu0/cpufreq/ entries directly.
Note added 2/28/07.  As of this date some distributions such as Mandriva 2007 do all
of this for you.  Moreover, they default to this mode.
Note added 8/20/08.  The configuration file /etc/sysconfig/cpufreq in RedHat
derived distributions may contain only commented out lines.  To enable power saving
uncomment the governor line and set it to ondemand.


Recent opterons seem to support the same power savings as Athlon 64. See above for info.

Note added 2/28/07. HOWEVER, there have been exceptions. I had the unfortunate experience recently of purchasing an Opteron 246HE CPU only to find that it supported only a single power level. Now the irony of this is that I bought the HE part because it is a low power part, but it turned out that AMD made 2 different chips, called "Opteron 246HE", and by bad luck the variant I ended up with did not actually support CoolN'Quiet. This information was well buried deep in an AMD technical manual. So when purchasing AMD chips it is apparently mandatory to dig a little deeper to verify that the EXACT variant being purchased supports the desired power saving features. To do this you will need the AMD part number. For instance: OSK246CMP5AU, which is, by the way, the variant you do NOT want, the 246HE that has multiple power states is actually OSK246FAA5BL.

Intel Processors

Pentium II

Not written yet

Pentium III

Not written yet


Not written yet

Pentium4 Mobile

Windows XP Instructions

Set the Power Options control panel applet to Minimal Power Management.

Linux Instructions

Try cpudyn. This requires that cpufreq also be working.

Core/Core Duo

One Dell machine was tested. Perhaps Enhanced Speed Step was not enabled in the BIOS? (Note: 2/28/07, on inspection there was no option to change this in the BIOS.) In any case, at idle the power consumption was 76W for both "Always On" and "Minimal Power Management". So the Core Duo was quite efficient at idle, it just wasn't clear if Enhanced Speed Step was alway on regardless of the power control panel setting, or if it was never on. In any case, it was clear that there was no control over it on this machine.


Not written yet

Macintosh G3 and G4 Desktops

There is apparently no way to decrease idle power consumption on these machines.

G5 desktop, G3,G4, and G5 laptops

These machines can slew their CPU frequency and voltage between a low value (low power consumption) and a high value (high power consumption). There are three settings, found under system preferences -> power savings -> options, which are:

  1.  Automatic:  The operating system increases power consumption
      to match the load.
  2.  Highest:  The computer always runs at the highest CPU frequency
      and voltage.
  3.  Lowest:  The computer always runs at the lowest CPU frequency
      and voltage.

In order to reduce power consumption at idle use Automatic on the desktops and on the laptops when these are running on the charger. When the laptops run on batteries it is sometimes useful to employ the Lowest setting in order to maximize the battery life. On laptops this option may be set independently for battery mode and charger mode. Processes with root privileges may also set these options from the terminal window using the pmset command.

iMac Intel Core processor

On OS X 10.4.8 the System information tool on the one iMac tested said "2 Ghz Intel core duo" without giving any measured CPU speed. The same machine brought up in Windows XP SP2 with bootcamp identified itself as "Genuine Intel (R) CPU T2500@2.00 GHz, 999Mhz." The different speed listed is consistent with Enhanced Speed Step being active. However power consumption under XP was 83 watts for the power settings "Home/Office Desk", "Always On", and "Minimal Power Management". This is unfortunately conflicting information, with the system claiming to be running at half speed, yet not actually drawing different amounts of power at different power settings. Under OS X 10.4.8 it ran at 73 watts and there appeared to be no setting to control power consumption in system preferences -> energy sevaings.

How then to explain the power differences between XP and OS X on the same machine at idle? I believe it is in part due to the handling of the integrated display. Under OS X there is a software brightness control and changing that varied power consumption from 60W to 85W. There was no such control under XP, so most likely in that OS the backlight is full on at all times, and this explains the observed idle power consumption.

So the jury is out on Enhanced Speed Step on this iMac. It's unclear if it was on and even less clear how to control it.

Saving Power at the Display

Display Introduction

On many working computers it is possible to disable part of the video circuitry and to power down the display in order to save power, without sacrificing the primary function of the machine. Most servers make little or no use of their consoles, often to the extent that no display is even attached to the computer's video output. Should such a machine have a display device that should certainly be in its lowest power state when not actively in use. Similarly, workstations which are running background jobs may be left unattended for long periods of time, during which there is typically no reason to maintain a display. Lastly, there are some protocols such as X11 which must remain active or the connection will terminate. For these the machine cannot be dropped into standby mode when the operator will be absent temporarily, but some power may still be saved by disabling the display.

It is important to note that universally computers determine whether or not the display is needed not by the activity of the display itself, but rather by the activity of the input devices the operating system associates without that display. Any events generated by the corresponding keyboard or mouse will be interpreted as indicating that the display is in use. This is of course because the computer has no other way of detecting the presence of a person viewing the display. It is important to note that some applications, such as watching a movie, do not generate these associated input events. If these do not disable the display shutdown logic themselves it must be done manually before the application starts. Conversely, undesired input events such as a keyboard button stuck down or excessive vibration near a mouse may keep the display from ever powering down, even though no user is present.

Most operating systems have some concept of a "screen saver". These date back to the early days of the CRT display, where a static display could literally burn an image into the phosphor on the CRT. So the original purpose of the screen saver was, literally, to save the screen, by preventing a static image from remaining too long on the display. This had nothing whatsoever to do with saving power. As time progressed and energy conservation was considered it was usually integrated with the existing screen saver functionality. When energy conservation is the goal no traditional screen saver should be used - at best these leave the display in a state of high energy consumption, at worst they also cause significant energy consumption at the CPU. In most cases the desired configuration is achieved by setting the activation time for "Display Disable" to be shorter than that for "Screen Saver start".

Control of the state of the display by the computer is generally based on VESA DPMS (Display Power Management Signaling). This method signals an analog display device which power state it should occupy by turning the H-sync and V-sync signals on or off. This results in four power states: on, standby, suspend, and off. For a CRT display these vary in the order shown from maximum to minimum energy consumption. However, for an LCD display there is often no difference between standby, suspend, and off. For instance, on a Gateway FPD1775W monitor, which normally consumes 15W when on, changing to any of the other three states first drops the power consumption on the display to 3W, then the display indicates "No Signal" and briefly pulses back up to 15W, then finally drops down to 0W. (Not actually zero, just less than the power meter employed could measure.) Power consumption in the video circuitry on the computer may differ in these three states, so for energy conservation purposes it is usually best to just use the Off state. For Displays driven by digital connectors a different control method known as DMPM (Digital Monitor Power Management) is employed. However, so far the operating systems have lumped this in with their DPMS control, so that the end user need not worry about whether an analog or digital connection to the display is used.

Lastly, there is a slight amount of power savings on the video circuitry in the computer when the display is placed into a DPMS off state. This is true when the computer is busy or idle. These savings are trivial when compared to the power saved by turning off the display. This was measured with a "Kill A Watt" power meter on a Linux workstation containing a Quadro FX1400 graphics card running under the Nvidia proprietary driver. The graphic load was varied by running pulsar -s, or not, on top of a an otherwise idle system with two xterm windows open. Note that the Quadro card continued to run even when the display was turned off, and the approximately 1W difference in power on the workstation between DPMS on and off probably represents power used in the analog output circuitry.

   DPMS\Graphics  busy idle
   on             127  77-78
   off            126  76

Windows Display control

Windows XP apparently uses DPMS SUSPEND or STANDBY for its power saving mode, see
Power consumption measured on various displays. For an LCD display these are probably equivalent in power consumption to DPMS OFF, but for a CRT they are not.

From the Administrator account:

Please also read the section Windows XP power settings. Unprivileged users may change their screensaver settings but may not override the power configuration set by the Administrator.

Mac Display control

Linux Display control

Under Linux DPMS control depends on what process is in charge of the display. If an X11 server is running the primary DPMS settings are usually placed in the X11 config file. The location of this file may vary, but /etc/X11/xorg.conf is common. Within this file DPMS is usually set like this (many unrelated options are also present, but are not shown here- use the command

for details on a particular linux distribution): Times shown are in minutes. Most X11 servers are configured with the default DPMS times indicated above, and the corresponding (redundant) option lines are not prsent in the configuration file.

The commands

show and set the current X11 DPMS settings - note that the times are in seconds. When set the new times are applied immediately and override the configuration file setting only until the X11 server restarts. show and set the current X11 DPMS settings - note that the times are in seconds. the final command Immediately sets the display into the dpms off state. Shaking the mouse or pressing a key will restore the display.

Ubuntu, and presumably some other distributions, run xscreensaver automatically when a session starts. xscreensaver keeps a copy of its own DPMS settings which override whatever was in the X11 configuration file, but only while xscreensaver is running. To configure the DPMS settings for xscreensaver:

Note, version 8.04.1 of Ubuntu has an issue with DPMS once the user logs out and the GDM login screen appears. When this happens xscreensaver is not running, and the DPMS values for the X11 server are apparently all zero, so that the display never powers down. This is true even if an explicit xset was used to set the DPMS times before logging out.

The other comon screen saver in Linux is xlockmore, which is typically used to prevent a second user from utilizing a workstation while the first one is away temporarily. To configure xlockmore to shut down the display instead of running a screen saver modify the file /etc/X11/app-defaults/XLock (or its equivalent on a particular system) to contain:

When any user issues the xlock command the display will be turned off. Note that there is a thirty second mandatory delay built into the xlockmore code, so that even though the configuration says the display should be shut down in one second, it will not actually take place until the worm screen saver has been shown for half a minute. The worm mode requires very little from the CPU or video card and so uses very little power for the short time it is running.

If an X11 server is not running then in theory one should be able to set up DPMS with:

However, in practice this does not always work - it did not on either a Centos 5.2 or a Mandriva 2008.1 system. If it works on your system, great, otherwise you are pretty much out of luck. This command will immediately put the display into power saving mode: Unfortuntely, hitting a key is not sufficient to turn it back on again. That requires an explicit command which is not the sort of thing one normally wants to type blind with a blanked display.

Technical information and raw data

BIOS settings

In general for power saving to work acpi must be enabled in the motherboard's BIOS. Some machines are also able to enable power saving at idle in their BIOS, however at this time this capability seems to be rare. Please refer to your motherboard manual for further information.

Windows XP power definitions

More or less verbatim from:

Processor control policy types

In Windows XP, the processor performance control policy is linked to the Power Scheme setting in the standard Power Options control panel applet.

  Policy      Description
  None        Highest performance state
  Constant    Lowest Performance state
  Adaptive    Performance state chosen according to demand
  Degrade     Lowest performance state, plus additional
               linear performance reduction as battery discharges

Processor control policy

The user selects the Power Scheme to be used, and Windows XP matches it with a processor control policy.

  Power scheme             AC power   DC power
  Home/Office Desktop      None       Adaptive
  Portable/Laptop          Adaptive   Adaptive
  Minimal Power Management Adaptive   Adaptive
  Maximize Battery Life    Adaptive   Degrade
  Presentation             Adaptive   Degrade
  Always On                None       None

Windows XP power settings

There are several ways to set or display the power configuration on an XP system, these are:

  Right click on the Desktop -> select Properties -> select Screen Saver tab -> click on Power... button
  Start -> select Control Panels -> select Power Options
  Start -> select Programs -> select Accessories -> select Command Prompt -> enter powercfg /query (etc.)

IMPORTANT: Normally in Windows XP the power setting for the Administrator account overrides that for unprivileged accounts. The settings for the Administrator also apply to the logon screen. Just to make things even more interesting Windows XP will show the wrong power setting information for all accounts other than Administrator. This occurs no matter which methods are used to display the power parameters. Typically other users see the "Home/Office Desktop" power scheme, no matter which power scheme is actually in use. Non-Administrator users will not be able to change this (incorrect) value, and will receive an "Access is Denied" message when they click on either Apply or OK after making power scheme changes.

Power consumption measured on various processors

These values were measured with a P3 International P4400 "Kill A Watt" meter. Measurements include everything in the case (CPU, disks, fans, graphics card) but do not include the monitor.

Processor           OS               State              Watts
Athlon XP 2000+     -                off                   4
Athlon XP 2000+     Linux (2.6.11)   idle (athcool on)    87
Athlon XP 2000+     Linux (2.6.11)   idle (athcool off)  145
Athlon XP 2000+     Linux (2.6.11)   busy                ???
Athlon XP 2000+     Windows XP SP2   idle (coolon off)   147
Athlon XP 2000+     Windows XP SP2   idle (coolon on)
                                       MSR 256           101
                                       MSR 128           108
                                       MSR  64           112
                                       MSR  32           116
                                       MSR  16           118
                                       MSR   8           121
Athlon XP 2000+     Windows XP SP2   standby              73
Athlon XP 2000+     Windows XP SP2   busy (rendering)    180

Athlon MP 2200+     Linux (2.6.19)   idle (athcool on)    78
Athlon MP 2200+     Linux (2.6.19)   idle (athcool off)  113
Athlon MP 2200+     Linux (2.6.19)   busy                135

Sempron 3100+       Windows XP SP2   idle (1.0Ghz,1.1V)   73
Sempron 3100+       Windows XP SP2   idle (1.8Ghz,1.3V)   79
Sempron 3100+       Windows XP SP2   idle (1.8Ghz,1.3V)   97a
Sempron 3100+       BIOS             menu (1.8Ghz,1.3V)   94
Sempron 3100+       Windows XP SP2   busy (stresstest)   105

Athlon 64 3700+     -                off                   2
Athlon 64 3700+     Windows XP SP2   idle ("always on")  100
Athlon 64 3700+     Windows XP SP2   idle ("min power")   80
Athlon 64 3700+     Linux (2.6.17)   idle (+cpufreq)      80  (1000 MHz)
Athlon 64 3700+     Linux (2.6.17)   idle (-cpufreq)     100  (2200 MHz)
Athlon 64 3700+     Linux (2.6.xx)   busy (1 cpuburn)    130
Athlon 64 3700+     Linux (2.6.xx)   busy (2 cpuburn)    130

Dual Opteron 248    -                off                   4
Dual Opteron 248    Linux (2.6.9)    idle (-cpufreq)     213
Dual Opteron 248    Linux (2.6.9)    busy (rendering)    243

Dual Opteron 280    -                off                   3
Dual Opteron 280    Linux (2.6.31)   idle (+cpufreq)     116
Dual Opteron 280    Linux (2.6.31)   idle (-cpufreq)     172
Dual Opteron 280    Linux (2.6.31)   busy (1 cpuburn)    157
Dual Opteron 280    Linux (2.6.31)   busy (2 cpuburn)    171
Dual Opteron 280    Linux (2.6.31)   busy (3 cpuburn)    213
Dual Opteron 280    Linux (2.6.31)   busy (4 cpuburn)    225b

AthlonII X2 240     -                off                   1
AthlonII X2 240     Linux (2.6.31)   idle (+cpufreq)      74
AthlonII X2 240     Linux (2.6.31)   busy (1 cpuburn)    113
AthlonII X2 240     Linux (2.6.31)   busy (2 cpuburn)    142
AthlonII X2 240     Windows 7        idle                 74
AthlonII X2 240     Windows 7        busy (1 cpuburn)    113
AthlonII X2 240     Windows 7        busy (2 cpuburn)    142

Pentium II 400 Mhz  -                off                   1
Pentium II 400 Mhz  Windows XP SP2   idle                 48
Pentium II 400 Mhz  Windows XP SP2   busy (booting)       65

Pentium III 500 Mhz -                off                   3
Pentium III 500 Mhz Windows XP SP2   idle                 37
Pentium III 500 Mhz Windows XP SP2   busy (booting)       60

Pentium 4 2.8 Ghz   Linux (2.6.x)    idle                 71
Pentium 4 2.8 Ghz   Linux (2.6.x)    busy (1 cpuburn)    139
Pentium 4 2.8 Ghz   Linux (2.6.x)    busy (2 cpuburn)    154
Pentium 4 2.8 Ghz   Linux (2.6.x)    busy (3 cpuburn)    154
Pentium 4 2.8 Ghz   -                off                   1
Pentium 4 2.8 Ghz   Windows XP SP2   idle                101
Pentium 4 2.8 Ghz   Windows XP SP2   busy (rendering)    139

Core Duo 1.86Ghz    -                off                   2                                      
Core Duo 1.86Ghz    Windows XP SP2   idle ("Always on")   76
Core Duo 1.86Ghz    Windows XP SP2   idle ("Min. power")  76

iMac G5 1.8 Ghz     OS X (10.3.9)    idle (highest)       91
iMac G5 1.8 Ghz     OS X (10.3.9)    idle (automatic)     85
iMac G5 1.8 Ghz     OS X (10.3.9)    idle (reduced)
                                       display on         85
                                       display off        45
iMac CoreDuo 2.0Ghz OS X (10.4.8)    idle (no control)    73
iMac CoreDuo 2.0Ghz Windows XP SP2   idle ("Always on")   83
iMac CoreDuo 2.0Ghz Windows XP SP2   idle ("Home/Office") 83
iMac CoreDuo 2.0Ghz Windows XP SP2   idle ("Min. power")  83

Details: Athlon XP 2000+. Asus A7V266E motherboard, 1G RAM, 1 40Gb IDE disk, ATI Radeon 8500DV graphics card.

Details: Athlon MP 2200+. Tyan S2466N motherboard, 1G RAM, 1 40Gb IDE disk, S3 graphics card.

Details: Sempron 3100+. AMD 754 socket, DFI LanParty UF nF3 250GB motherboard, 512M RAM, 1 80Gb IDE disk, ATI Radeon 9200SE.

Details: Athlon 64 3700+. Asus A8N5X motherboard, 1G RAM, 1 160Gb IDE disk, Nvidia Quadro Pro FX 1400 graphics card.

Details: Core Duo 1.86 Ghz. Dell Precision 390, unknown components. Thank you to Leonard Thomas for providing access to this machine

Details: Imac Core Duo 2.0 Ghz. Apple iMac, unknown components. Thank you to Anthony West for providing access to this machine

Details: Opteron 280 2.4 GHz Arima HDAMAI motherboard, dual dual core processors, 4G Ram, 1 40GB SATA disk, on board low end graphics (not active), dual broadcom NICs at 100baseT speed.

Details: AthlonII X2 240 "Regor" 2.8 GHz Gigabyte MA785GMT-UD2H , dual core processor, 2G DDR3 Ram, 1 TB SATA disk, Nvidia Quadro Pro FX 1400 graphics card.

Details: Pentium 4 2.8 GHz Dell Optiplex GX270, 1GB RAM.

a Halt also disabled using CBID tool.

b cpufreq enabled, governor=ondemand, sched_mc_power_savings set.

Power consumption measured on various displays

These values were measured with a P3 International P4400 "Kill A Watt" meter. Measurements include only the monitor. Note that for both the conventional Iiyama CRT and the Dell LCD flatpanel the power consumption decreases with brightness. Turning off half the lights in an office (1-8 32W fluorescent bulbs) allows the display to be run at a lower brightness. This saves power on the display, on the room lighting, and in most cases on the building air conditioning. Data with explicit DPMS states was measured using the Linux vbetool.

Display              State                          Watts
Iiyama V.Master 450  DPMS ON                        92
Iiyama V.Master 450  DPMS SUSPEND,STANDBY            6
Iiyama V.Master 450  DPMS OFF                        3
Iiyama V.Master 450  Windows XP "monitor off"        6
Iiyama V.Master 450  640x480 (BIOS display)         64
Iiyama V.Master 450  1280x1024,75hz,bright=100%     88
Iiyama V.Master 450  1280x1024,75hz,bright=50%
                        Mostly White                93
                        Typical screen              84
                        Mostly Black                81
Iiyama V.Master 450  1280x1024,75hz,bright=25%      88

Dell 1801 LCD        Dimmest (unreadable)           18
Dell 1801 LCD        30% Brightness (as used)       35
Dell 1801 LCD        Brightest (blinding)           50

Viewsonic VX922      Normal use                     30
Viewsonic VX922      screen saver - blank           30
Viewsonic VX922      "monitor off" (Linux or XP)     0

Gateway FPD1775W     DPMS ON                        15
Gateway FPD1775W     DPMS OFF,SUSPEND,STANDBY        0

Power consumption measured on various screen savers

These values were measured with a P3 International P4400 "Kill A Watt" meter. Measurements include everything in the case (CPU, disks, fans, graphics card) but do not include the monitor. The interesting point here is that the screen savers may or may not push a machine out of its idle mode, depending on the CPU, and possibly also on the graphics card. The results shown below are for Windows XP screen savers. All numeric values are in watts.

Athlon XP 2000+   Sempron 3100+     Pentium4 (2.8Ghz)   Screen Saver
108               73                101                 Windows XP
150               74                115                 3D Flowerbox
147               74                -                   3D Flying Objects
110               -                 115                 3D Pipes
147               -                 -                   3D Text
111               -                 -                   Beziers
108               73                101                 blank
108               -                 -                   Marquee
115               -                 -                   Mystify
115               -                 -                   Starfield

Power consumption measured on various printers

These values were measured with a P3 International P4400 "Kill A Watt" meter.

Printer           State             Power
Genicom ml260     power saver       16
Genicom ml260     on                16 (spiking periodically to 620)
Genicom ml260     warming up        620

Genicom cl160     power saver       38
Genicom cl160     on                38 (spiking periodically to 650)
Genicom cl160     warming up        650
Genicom cl160     printing          650-750

HP 2100M          power saver/on    22
HP 2100M          warming up        600
HP 2100M          printing          400-430

Calculating power costs

Assume a typical employee works 8 hours a day, 5 days a week, and has two weeks of vacation per year. Furthermore assume that this employee's equipment is turned off when the employee is not present. For each watt consumed by that employee's equipment in a year 8*5*50 = 2000 watt hours = 2 kilowatt hours of energy must be purchased. In California as this is written the cost per kWh is about ten cents. Resulting in the formula:

Server equipment is typically on all the time. So for power consumption in servers allow 24 hours a day, 365 days a year. For each watt consumed by the server equipment in a year it will consume 24*365 = 8760 watt hours = 8.8 kilowatt hours of energy. Assuming again 10 cents per kWh this would cost 88 cents. Resulting in the formula:

Finally it is worth considering the costs of giving way idle CPU time. This practice was pioneered by SETI and there are now several other projects which run on the same BOINC software. A clerical worker's PC is typically idling nearly 100% of the time since the relevant applications demand very little of the CPU. Installing SETI changes this equation. First, the background program consumes the idle cycles and moves the machine from its lowest power state to a higher power busy state. Secondly, and this is actually the bigger factor by far, the worker may leave the machine on all the time in order to let the background program run.

Case 1 (least expensive). The typical office worker, using the Athlon XP 2000+ machine described elsewhere, with the cooling mode enabled, and an LCD screen. Average power consumption would then be 108W for the elements in the case plus 35W for the display giving 143W total. Using the first formula above gives a yearly power cost of $28.60.

Case 2. The same office worker, using the same machine, but this time with a SETI like program using all idle cycles, but not heavily loading the machine. The machine is now left on 24 hours a day as it's "doing something worthwhile". Average power consumption for the box becomes 147W, and since the machine now runs all the time yearly energy cost for these components is 147*.88 = $129.36. Presumably the user would still turn the monitor on and off as before, so the yearly energy cost for the display would 35*.2 = $7.00. Giving a total energy cost of $136.36.

Case 3 (most expensive). The same office worker, using the same machine, but this time with a SETI like program pushing the CPU hard. Average power consumption for the box becomes 180W, and since the machine now runs all the time yearly energy cost for these components is 180*.88 = $158.40. Presumably the user would still turn the monitor on and off as before, so the yearly energy cost for the display would 35*.2 = $7.00. Giving a total energy cost of $165.40.

Replace CRTs with LCDs to save money?

The same office worker, using an Iiyama CRT will use about 84W, which will cost 84*.2 = $16.80 for yearly energy costs. As noted above, an LCD would use about 35W and so would cost 35*.2 = $7.00 for the year. Assuming the replacement LCD display cost $200 the payback period would be 20 years. (Note added 2/28/07 - this argument is still technically correct but moot since pretty much all new displays are LCDs. The normal turnover of displays has replaced almost all desktop CRTs at this point.)

Blanking screen vs. turning off displays

LCDs can go "black" either by shutting off the backlight (shutting down the display) or by turning all pixels to the black state. The problem is, while both result in a black display, the former results in significant energy savings, and the latter saves absolutely nothing. For this reason, if given the option, screen savers should be avoided and the monitor turned off instead. On Windows XP this is accomplished by setting the screen saver time longer than the Monitor Power time in the Display Properties control panel. For instance, a Viewsonic VX922 19" display uses 30W when blanked. When the OS shuts off the display it shows "No signal" for a few seconds. While doing that it also uses 30W. Then once the display goes into standby mode it uses 0W.

Links to software
AMD's drivers and tools are listed by processor here:
Central Brain Identifier (aka cbid)

Commercial software - (Link sent by manufacturer) for Windows. Site management of power usage.


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Copyright David Mathog and Caltech 2009


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Updated 5/10/2010