Reduce Your PC's Power and Operating Costs

Save as Your PC Sleeps

Modern PCs incorporate a power-management standard called the Advanced Configuration and Power Interface (ACPI), which allows the operating system, BIOS, and hardware to cooperate in reducing power consumption. ACPI defines several global power and sleeping states; our chart below lists the relevant ACPI states and corresponding Windows XP/Vista modes, along with the power used by each mode on the example system.

You've probably put your laptop into hibernation or sleep hundreds of times, but people typically ignore these features on desktops. As the chart shows, power use in the S3 state (Standby in XP, and Sleep in Vista) is nearly equal to that used when the PC is powered off. The S3 state drops PC power down to 10 watts or less when idle even if the system consumes considerably more power than average while active. Unfortunately, most PCs I encounter are not configured to use this mode. Some older boards and peripherals may not play nice with S3, sound cards being particularly troublesome.

To tweak power management settings in Windows XP and Vista, open the Power Options applet in Control Panel. The three preset schemes are:

  • Turn off monitor and/or hard disks only
  • Standby/Sleep mode
  • Hibernate mode

While Hibernate mode uses the least amount of power, it takes the longest time to suspend and resume, which involves writing and reading the entire contents of RAM to and from the hard disk (using a file named hiberfil.sys). The more RAM you have, the longer the process takes. On the other hand, the Standby/Sleep mode (S3) uses only slightly more power than Hibernate does, yet its suspend and resume times are virtually instantaneous no matter how much RAM is installed.

A BIOS setting called ACPI Suspend Type, ACPI Suspend State, or something similar controls which sleep state (S1 or S3) Windows uses. Many systems are set to S1 by default, which keeps the CPU and RAM powered. Enter your BIOS, navigate to the Power Management menu, and change the ACPI Suspend setting to S3, which cuts power to the CPU and RAM. The screen shot on the first page shows this setting on a Phoenix/Award BIOS. Enable any settings labeled 'USB KB Wake-Up From S3' (the wording varies), and set the Power On Function to Any Key to let the keyboard and mouse wake the PC from standby.

Change Settings to Conserve PC Energy

Depending on the Advanced Configuration and Power Interface (ACPI) settings in your PC's BIOS, the machine may be running up your energy bill even when you're not using it. Click the icon below to see our chart describing different levels of power consumption.

Change Settings to Conserve PC Energy
Depending on the Advanced Configuration and Power Interface (ACPI) settings in your PC's BIOS, the machine may be running up your energy bill even when you're not using it.
PC ACTIVITY ACPI Global/Sleep State Windows XP/Vista state Typical power consumed
100 percent CPU/graphics load G0 Power on Running 305W
3D screen saver G0 Power on Running 225W
Simple screen saver G0 Power on Running 195W
Idle; Windows desktop G0 Power on Running 195W
Monitor and hard drives powered down G0 Power on Blank screen 160W
Monitor and hard drives powered down, CPU halted, fans and other devices running G1/S1 Power on suspend (POS) Standby/Sleep 135W
Context saved in RAM, everything except RAM powered off G1/S3 Suspend to RAM (STR) Standby/Sleep 10W
Context saved to disk (hiberfil.sys), system powered off G1/S4 Suspend to disk (STD) Hibernate 9W
System powered off G1/S5 Soft-off Shut Down 9W
System unplugged G3 Mechanical off Shut Down 0W

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