Shutdown, Hibernate or Sleep?
PowerMAN supports multiple power saving methods. Each has advantages and disadvantages. In some cases, they can be combined to create
the best compromise between usability and power saved or implement a different strategy for different users or different times of the day. Broadly, the methods are:
- Sleep (S0ix, S1-S3) – The computer appears off but the user session can be quickly be resumed in just a second or so. This is sometimes called "suspend to RAM" and requires the power source to be maintained. The user session is maintained and they may continue immediately after the system has resumed.
- Hibernate (S4) – The computer appears off but the user session can typically be resumed in 10-30 seconds. This is sometimes called "suspend to disk". Unlike sleep, this does not require the power source to be maintained and may be better suited to long term use. The user session is maintained and they may continue immediately after the system has resumed.
- Shutdown (Power Off) – The computer is turned off and requires a full start-up sequence to resume operation. The user session is not maintained and the user must login and start applications again to continue working.
In approximate order of energy saving, the individual power states are as follows:
- S1 Sleep (Smaller saving) - System appears off. The CPU is stopped; RAM is refreshed; the system is running in a low power mode. To maintain the user’s session the power supply must remain connected. The system can resume in a few seconds. This mode is rarely available on modern systems. In some cases, it may be enabled via a BIOS setting.
- S2 Sleep - System appears off. The CPU has no power; RAM is refreshed; the system is in a lower power mode than S1. To maintain the user’s session the power supply must remain connected. The system can resume in a few seconds. This mode is rarely available on modern systems
- S3 Suspend (Bigger saving) - System appears off. The CPU has no power; RAM is in slow refresh; the power supply is in a reduced power mode. To maintain the user’s session the power supply must remain connected. The system can resume in a few seconds. This is the most common power saving mode on modern systems. In some cases, it must be enabled via a BIOS setting.
- S0ix "Modern" Standby - The system appears off but selected software may periodically run. To maintain the user’s session the power supply must remain connected. The system can resume instantly. If this mode is supported, it will consume a similar amount of energy to S3 but provide more software functionality. If S0ix is present, the S1-S3 modes will not be available. This feature is only available from Windows 8 and later on some high-end devices.
- S4 (Hibernate) (Bigger saving) - System is turned off. The CPU and RAM have no power. The power supply may be disconnected without any loss to the user’s session. The system can typically resume in 10-30 seconds.
The difference between the S4 and S5 states is that the computer can resume to the previous user session from S4. This is not possible from the S5 state. NB: Whilst the S4 state should consume less energy than S3, the difference is often very small (and sometimes practically unmeasurable). The main reason to select S4 is because it protects the device against power failure (at the expense of a longer resume time) and not because it may use less energy.
- S5 (Shutdown) - System is turned off. There is no user session. A full start-up sequence is required to use the device. This may take 20-60 seconds or more.
NB: A system in the shutdown state may still consume some energy if connected to the mains supply. In most systems there is little measurable difference in power consumption between sleep, hibernate and shutdown. In some cases, enabling Wake-on-LAN has been observed to increase S4/S5 power consumption over that of S3.
Most modern systems support either S3 or S0ix "Modern" Standby. Support for the S1/S2 modes is now very rare. In practice, there is often little difference in energy consumption between S1-S4. The choice to use either sleep or hibernate is usually based upon the reliability of the power source and the resume time.
Tip: Hibernate and sleep do not log out the current user. Any programs or open documents are preserved and the user may continue immediately after the system has resumed.
In a shared computer environment, this can result in the next user being unable to logon because the workstation is locked by the previous user. Consequently, these approaches are not appropriate for systems in shared areas. This can be avoided by logging out idle users and then performing power saving when no user is logged on.
Please remember that not all states will be available on all hardware. The POWERMAN INFO command or the PowerMAN reports can be used to get detailed technical information about which states are available on a specific PC. As a general rule, PowerMAN will use the most energy efficient sleep state (S0ix, S1-S3) supported by the hardware.