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Power Cycling

Power Supply Reference: Consumption, Savings, And More
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Should you turn off a system when it is not in use? To answer this frequent question, you should understand some facts about electrical components and what makes them fail. Combine this knowledge with information on power consumption, cost, and safety to come to your own conclusion. Because circumstances can vary, the best answer for your own situation might be different from the answer for others, depending on your particular needs and applications.

Frequently powering a system on and off does cause deterioration and damage to the components. This seems logical, but the simple reason is not obvious to most people. Many believe that flipping system power on and off frequently is harmful because it electrically “shocks” the system. The real problem, however, is temperature or thermal shock. As the system warms up, the components expand; as it cools off, the components contract. In addition, various materials in the system have different thermal expansion coefficients, so they expand and contract at different rates. Over time, thermal shock causes deterioration in many areas of a system.

From a pure system-reliability viewpoint, you should insulate the system from thermal shock as much as possible. When a system is turned on, the components go from ambient (room) temperature to as high as 185°F (85°C) within 30 minutes or less. When you turn off the system, the same thing happens in reverse, and the components cool back to ambient temperature in a short period.

Thermal expansion and contraction remains the single largest cause of component failure. Chip cases can split, allowing moisture to enter and contaminate them. Delicate internal wires and contacts can break, and circuit boards can develop stress cracks. Surface-mounted components expand and contract at rates different from the circuit boards on which they are mounted, causing enormous stress at the solder joints. Solder joints can fail due to the metal hardening from the repeated stress, resulting in cracks in the joint. Components that use heatsinks, such as processors, transistors, or voltage regulators, can overheat and fail because the thermal cycling causes heatsink adhesives to deteriorate and break the thermally conductive bond between the device and the heatsink. Thermal cycling also causes socketed devices and connections to loosen, or creep, which can cause a variety of intermittent contact failures.

Thermal expansion and contraction affect not only chips and circuit boards, but also things such as hard disk drives. Most hard drives today have sophisticated thermal compensation routines that make adjustments in head position relative to the expanding and contracting platters. Most drives perform this thermal compensation routine once every five minutes for the first 30 minutes the drive is running and then every 30 minutes thereafter. In older drives, this procedure can be heard as a rapid “tick-tick-tick-tick” sound.

In essence, anything you can do to keep the system at a constant temperature prolongs the life of the system, and the best way to accomplish this is to leave the system either permanently on or permanently off. Of course, if the system is never turned on in the first place, it should last a long time indeed!

Now, I am not saying that you should leave all systems fully powered on 24 hours a day. A system powered on when not necessary can waste a tremendous amount of power. An unattended system that is fully powered on can also be a fire hazard. (I have witnessed at least two CRT monitors spontaneously catch fire—luckily, I was there at the time.)

The biggest problem with keeping systems on 24/7 is the wasted energy. Typical rates are 10 cents for a kilowatt-hour of electricity. Using this figure, combined with information about what a typical PC might consume, we can determine how much it will cost to run the system annually and what effect we can have on the operating cost by judiciously powering off or taking advantage of the various ACPI Sleep modes that are available. ACPI is described in more detail later in this chapter.

A typical desktop-style PC consumes anywhere from 75 W to 300 W when idling and from 150 W to 600 W under a load, depending on the configuration, age, and design of the system. This does not include monitors, which for LCDs range from 25 W to 50 W while active, whereas CRTs range from 75 W to 150 W or more. One PC and LCD display combination I tested consumed an average of 250 W (0.25 kilowatts) of electricity during normal operation. The same system drew 200 W when in ACPI S1 Sleep mode, only 8 W while in ACPI S3 Sleep mode, and 7 W of power while either turned off or hibernating (ACPI S4 mode).

Using those figures, here are some calculations for annual power costs:

Electricity Cost:    $0.10 Dollars per KWh
PC/Display Power:    0.250 KW avg. while running
PC/Display Power:    0.200 KW avg. while in ACPI S1 Sleep
     PC/Display Power:    0.008 KW avg. while in ACPI S3 Sleep
    PC/Display Power:    0.007 KW avg. while in ACPI S4 Sleep
    PC/Display Power:    0.007 KW avg. while OFF
Work Hours:     2080 Per year
Non-Work Hours:     6656 Per year
     Total Hours:     8736 Per year
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Annual Operating Cost:  $218.40 Left ON continuously
Annual Operating Cost:  $185.12 In S1 Sleep during non-work hours
Annual Operating Cost:   $57.32 In S3 Sleep during non-work hours
Annual Operating Cost:   $56.66 In S4 Sleep during non-work hours
Annual Operating Cost:   $56.66 Turned OFF during non-work hours
-------------------------------------------------------------------
       Annual Savings:    $0.00 Left ON continuously
       Annual Savings:   $33.28 In S1 Sleep during non-work hours
   Annual Savings:  $161.08 In S3 Sleep during non-work hours
   Annual Savings:  $161.74 In S4 Sleep during non-work hours
Annual Savings:  $161.74 Turned OFF during non-work hours


This means it would cost more than $218 annually to run the system if it were left on continuously. However, if it were turned off during nonwork hours, the annual operating cost would be reduced to $56, for an annual savings of more than $161! As you can see, turning systems off when they are not in use can amount to a huge savings over time.

But even more interesting is that you don’t have to turn a system all the way off to achieve this type of savings. When properly configured, most PCs will enter ACPI S3 Sleep mode either manually or after a preset period of inactivity, dropping to a power consumption level of 8W or less. In other words, if you configure the PC to enter S3 Sleep mode when it’s not active, you can achieve nearly the same savings as if you were to turn it off completely. In the preceding example, it would only cost an additional $0.66 to keep the system in Stand By mode during nonwork hours versus turned completely off, still resulting in an annual savings of more than $161.

With the improved power management capabilities of modern hardware, combined with the stability and control features built into modern OSs, systems can Sleep and Resume almost instantly, without having to go through the lengthy shutdown and cold boot startup procedures over and over again. I’m frankly surprised at how few people I see taking advantage of this because it offers both cost savings and convenience.

Many people perform a full shutdown procedure when turning off their computer, closing all open applications, shutting down the OS and system completely. Then when powering back on, they do a cold boot and reload the OS, drivers, and applications from scratch.

There is an alternative that is much better. Instead of shutting down completely, put the system to Sleep instead. When in Sleep mode the system saves the full system context (state of the system, contents of RAM, and so on) in RAM before powering off everything but the RAM. Unfortunately, many systems aren’t configured to take advantage of Sleep mode, especially older ones. Note that Sleep was called Standby (or Stand by) in Windows XP and earlier.

The key is in the system configuration, starting with one important setting in the BIOS Setup. The setting is called ACPI suspend mode, and ideally you want it set so that the system will enter what is called the S3 state. S3 is sometimes called STR for Suspend to RAM. That has traditionally been the default setting for laptops; however, many if not most desktops unfortunately have ACPI suspend mode set to the S1 state by default. ACPI S1 is sometimes called POS for Power on Suspend, a state in which the screen blanks and CPU throttles down; however, almost everything else remains fully powered on. As an example, a system and LCD display that consumes 250W will generally drop to about 200W while in S1 Sleep; however, the same system will drop to only 8W of power consumption in the S3 (Suspend to RAM) state.

When the system is set to suspend in the S3 state, upon entering Sleep (either automatically or manually), the current system context is saved in RAM and all the system hardware (CPU, motherboard, fans, display, and so on) except RAM is powered off. In this mode, the system looks as if it is off and consumes virtually the same amount of power as if it were truly off. To resume, you merely press the power button just as if you were turning the system on normally. You can configure most systems to resume on a key press or mouse click as well. Then, instead of performing a normal cold boot and full restart, the system almost instantly powers on and resumes from Sleep, restoring the previously saved context. Your OS, drivers, all open applications, and so on, appear fully loaded just as they were when you “powered off.”

As mentioned, many people have been using this capability on laptops, but few seem to be aware that you can use it on desktop systems also. To enable this deeper sleep capability, there are only two main steps:

  1. Enter the BIOS Setup, select the Power menu, locate the ACPI suspend setting, and set it to enter the S3 state (sometimes called STR for Suspend to RAM). Save, exit, and restart.
  2. In Windows, open the Power Options tool in the Control Panel, locate the setting for the Power button and change it to Sleep or Stand by.

You can also take advantage of hibernation, which allows you to use the ACPI S4 (STD = Suspend to Disk) state in addition to S3. ACPI S4 is a lot like S3, except the system context is saved to disk (in a file called hiberfil.sys) instead of RAM, after which the system enters the G2/S5 state. The G2/S5 state is also known as Soft-Off, which is exactly the same as if the system were powered off normally. When you power on from Hibernation (S4), the system still cold boots; however, rather than reloading from scratch, Windows restores the system context from disk (hiberfil.sys) instead of rebooting normally. Although hibernating isn’t nearly as fast as S3 (Suspend to RAM), it is still much faster than a full shutdown and restart and works even if the system loses power completely while suspended. Windows XP and earlier allows you to place a system in Standby (Sleep) or Hibernate modes, while Windows Vista and later has Sleep, Hibernate, and Hybrid Sleep modes. Hybrid Sleep is a combination of sleep and hibernate, where the system state is saved both in RAM and to the hard disk as a backup. Hybrid Sleep is the default Sleep function setting for desktop systems, and because of the extra time to create the hiberfil.sys file it unfortunately makes the system take just as long to Sleep as it does to Hibernate. To speed up the Sleep mode functionality in Windows 7/Vista you can disable
Hybrid Sleep.

Finally, to make the system Sleep automatically, you can change the Windows Power Scheme settings to put the system in Sleep mode after a time duration of your choice. This allows the system to automatically enter Sleep mode after the preset period of inactivity (I usually set it for 30 minutes to an hour) has elapsed.

By using S3 Sleep mode, you can effectively leave the system running all the time yet still achieve nearly the same savings as if you turned it off completely. Servers, of course, should be left on continuously; however, if you set the system to Wake on LAN (WOL) in both the BIOS Setup and in Windows, the system can automatically wake up anytime it is being accessed. The bottom line is that taking advantage of Sleep mode can save a significant amount of energy (and money) over time.

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