To the user, the system appears to be either on or off. There are no other detectable states. However, the system supports multiple power states that correspond to the power states defined in the Advanced Configuration and Power Interface (ACPI) specification. The following table lists the power states from highest to lowest power consumption.
|Power state||ACPI state||Description|
|Working||S0||The system is fully usable. Devices that are not in use can save power by entering a lower power state.|
|The system appears to be off. Power consumption is reduced to one of several levels, depending on how the system is to be used. The lower the level of power consumption, the more time it takes the system to return to the working state.|
|Hibernation||S4||The system appears to be off. Power consumption is reduced to the lowest level. The system saves the contents of memory to a hibernation file, preserving the state of the operating system, applications, and open documents.|
|Soft Off||S5||The system appears to be off. Some components remain powered so the computer can wake from input from the keyboard, LAN, or a USB device. The working context can be restored if it is stored on nonvolatile media.|
|Mechanical Off||G3||The system is completely off and consumes no power. The system returns to the working state only after a full reboot.|
The SYSTEM_POWER_STATE enumeration defines values that are used to specify system power states.
The system enters sleep based on a number of criteria, including user or application activity and preferences that the user sets in the Power Options application in Control Panel. By default, the system uses the lowest-powered sleep state supported by all enabled wake-up devices. For more information about how the system determines when to enter sleep, see System Sleep Criteria.
Before the system enters sleep, it determines the appropriate sleep state, notifies applications and drivers of the pending transition, and then transitions the system to the sleep state. In the case of a critical transition, such as when the critical battery threshold is reached, the system does not notify applications and drivers. Applications need to be prepared for this and take the appropriate action when the system returns to the working state.
Windows Server 2003 and Windows XP:
By default, the system uses the lowest sleep state supported by all enabled wake-up devices within the specified latency requirements. To set the default sleep state, the user can use the Power Management application in Control Panel. An application can specify its latency requirements using the RequestWakeupLatency function.
After the system selects the sleep state the system will enter, it requests permission to enter the sleep state from all applications, using the PBT_APMQUERYSUSPEND event. If no applications deny the request, the system broadcasts it to the drivers by using IRP_MJ_POWER and IRP_MN_QUERY_POWER. If any drivers deny the request, the system requests the next highest sleep state until a sleep state is successfully entered or there are no more available sleep states.
The system wakes from sleep in response to user activity or a wake-up event defined by an application. For more information, see System Wake-up Events. The amount of time it takes the system to wake depends on the sleep state it is waking from. The system takes more time to wake from a lower-powered state than from a higher-powered state because of the extra work the hardware may have to do (stabilize the power supply, reinitialize the processor, and so forth). The system takes the most time to wake from hibernation (S4) because it must read the hibernation file.
The following features can affect the sleep state if they are enabled in the user's active power plan:
- If hybrid sleep is enabled, the system writes a hibernation file but enters a higher-powered sleep state. If power is maintained while the system is sleeping, the system wakes from the sleep state. If power is lost while the system is sleeping, the system wakes from hibernation, which takes longer but restores the user's system state.
- Away mode enables the computer to appear to be sleeping while it remains in the working state (S0) to process background media tasks such as streaming media to other devices or recording television content. An application requests away mode by using SetThreadExecutionState with ES_AWAYMODE_REQUIRED and ES_CONTINUOUS.
Important Away mode should be used only by media applications and only when absolutely necessary, because it prevents the system from conserving power by entering a true sleep state. This can have a negative impact on portable computer battery life and scenarios that depend on effective power management, such as large datacenters.
Windows Server 2003 and Windows XP:
Hybrid sleep and away mode are not supported.
The wake-on-LAN (WOL) feature wakes the computer from a low power state when a network adapter detects a WOL event (typically, a specially constructed Ethernet packet). Behavior in response to WOL events has changed from Windows 7 to Windows 8.
Windows 7: In Windows 7, the default shutdown operation puts the system into classic shutdown (S5) and all devices are put into the lowest power state D3. Wake-On-LAN is not officially supported from S5 in Windows 7. However, some network adapters can be left armed for wake if enough residual power is available. As a result, wake from the S5 state is possible on some systems where enough residual power was supplied to the NIC even though the system is in S5 and devices are in D3.
Windows 8: In Windows 8, the default shutdown behavior puts the system into hybrid shutdown (S4) and all devices are put into D3. Remote wake from hybrid shutdown (S4) or classic shutdown (S5) is completely unsupported. In Windows 8, NICs are explicitly not armed for wake in both the classic shutdown (S5) and hybrid shutdown (S4) cases because users expect zero power consumption and battery drain in the shutdown state. This behavior removes the possibility of spurious wakes when explicit shutdown was requested. As a result, Wake-On-LAN is only ever supported from sleep (S3) or hibernate (S4) in Windows 8.
Note that in Windows 8, hybrid shutdown (S4) stops user sessions but the contents of kernel sessions are written to hard disk. This enables faster boot.