Home / Projects / ENERGY STAR /

System Standby (S3) v. Hibernation In New Windows PCs

Background

Before understanding which mode of PC power management is best for a particular situation, it is important to have a basic understanding of the similarities and differences between System Standby and Hibernation in modern computers1.

System standby is a hardware/software solution where the hardware and software work together to manage the computer's power usage by shutting down some non-critical systems (Fans, drives, peripherals) and throttling back the critical ones (CPU, Network). Hibernation is a Software only solution that basically takes the current snapshot of the system (RAM memory) and writes it to the hard disk then informs the hardware that it should do a complete shutdown. When the hardware is turned back on, the hardware does a full POST (Power On/Self Test) process and then passes off to the software to do a full boot. Since the software wrote a copy of itself to the hard drive it skips it's full boot process and simply loads the system snapshot written to disk back into RAM and the applications that were running generally have no idea they were not running during that period of time the machine was "off". In short, Hibernation was created to give the false impression of quick "boot" times for users who wanted to start up their computers faster.

All of this seems to suggest that Hibernation would save more energy than System Standby. Although more will be said later on with regards to the subject, it is important to understand how modern PC hardware is designed. In the "old" days2 the power switch was hardwired to the power supply and when the switch was off then the power supply was off and little3 energy was being consumed. In modern configurations4 the power supply is always drawing some energy and supplying the system's motherboard with a trickle current. The motherboard uses this to keep an idea of what state the hardware is in ("on" or "off") and manage the devices accordingly. When a user switches "on" the power button, the motherboard5 determines what state it should change to based on rules. In a two state system the most basic is what state is the hardware in now and then change to the opposite state. As such, Hibernation, System Standby and turning a system "off" will result in similar if not the same power draws.

Analysis

In general we recommend the application of system standby over hibernation for the following reasons.

  1. Typically system standby can be set more aggressively (i.e. less than 30 minutes) because of a quicker wake up time - thus saving more energy. System standby keeps RAM powered rather than writing the system's current RAM image to the hard drive and thus responds quicker to wake up events. For comparison, typical hibernate settings are 60 - 120 minutes.
  2. System Standby allows the computer to wake and respond to modem, peripheral and network interrupt requests while hibernation does not allow such events to trigger the machine to come up from a hibernation state. Hibernation does allow a Wake-On-LAN (WOL) network event to bring the system back up from the "off" state but this WOL signal requires special administrative tools network administrators typically use to centrally manage PCs.
  3. Typically system standby requires keyboard or mouse activity for a wake up, while hibernate requires a push of the power button. As with monitor power management, a keyboard or mouse wake up the computer, therefore it is easier for users to get accustomed to system standby. With hibernate, users have to first tap the keyboard to see if only monitor power management is engaged, and then push the power button to wake from hibernate.
  4. If data retention, in say the face of a power outage, is desired then hibernate can be set in addition to system standby. For example, a power outage knocks out two machines. One in standby and the other in hibernation. Any unsaved files on the standby machine may be lost while anything on the hibernating machine will be saved since the RAM has been written to disk.
    The following power management configuration would constitute an "aggressive" policy for energy savings but minimize wake up time and provide minimal data retention protections.
    1. Monitor: 5 minutes
    2. System Standby: 15 minutes
    3. Hibernate: 60 minutes

  5. Continually placing a system in Hibernation means that the system stays loading without being refreshed from the disk. Although operating systems like Windows 2000 and XP have improved in their stability, it is still better to periodically reboot them to avoid crashes. Utilizing System Standby and shutting down overnight forces this periodic shutdown and makes for a better user experience.

These suggestions are based on recent standard PC configurations. For example, the table below summarizes measurements taken on a Dell Dimension 4400, Pentium 4.

System Standby (S3) Hibernate (S4) Notes

Energy Savings Reduces power consumption from about 50 to 2.5 watts. Saving 47.5 watts. Reduces power consumption from about 50 to 1.8 watts. Saving 48.2 watts. Effectively no difference in energy savings potential between system standby and hibernate.

Response Time 5 - 10 seconds More than 20 seconds The response time for hibernate depends on the amount of memory that was saved on the hard disk during the shutdown procedure.
Saves work in the event of a power loss? No Yes System hibernate may be best applied to laptops that are running on batteries.

Response Mechanism Keyboard or mouse Power button With system standby users only need one mechanism (keyboard mouse) for both monitor and PC. Note: This may vary from brand to brand.

Typical Sleep Settings Less than 30 minutes to be qualified for ENERGY STAR. Greater than 60 minutes Shorter sleep settings means more energy savings.

Conclusion

The choice to enable System Standby, Hibernation or turning the system off is as much based on the situation the computer is used in as it is on power savings. Using the above information, one should be able to find a suitable method for their given situation. In most cases it is best to enable monitor power management at a period of 10 minutes, system standby at a time of 20 to 30 minutes. Hibernation should be employed at 60 minutes or higher if one wants their machine in a deep sleep for long periods of time and who is also concerned with start up times after long periods of being in the "off" state.

Footnotes

  1. Modern applies to Pentium 4 or higher IAPC enabled machines and equivalents (OnNow).
  2. Pentium and below or below and equivalents. AKA: AT form factor
  3. Some energy would get through in what is known as Vampire draw.
  4. Approximately Pentium 2 or higher and equivalents AKA: ATX form factor and above NB: that IAPC is only available in Pentium 4 or higher and equivalents.
  5. More specifically the Basic Input/Output System (BIOS)