ACPI defines several power states. Depending on the support of your motherboard, BIOS and OS, some of these may not be available to you:

• G0: Awake. The system is fully powered up and all devices function. No power is saved.
• G1. The system is in one of the following states:
  • S1: Standby. The CPU is throttled down and some devices are powered down. The system state is maintained in memory. The system wakes almost instantly, but only a small amount of power is saved.
  • S2: (confusingly, also) Standby. As S1, but the CPU is powered down along with the devices. Power consumption is lessened (a lot less for a greedy AMD or Intel system), but waking takes a few seconds.
  • S3: Suspend to RAM. The processor and most devices are powered down; only the system state is maintained in RAM. Depending on the hardware in use, the computer can appear powered down (eg, fans can stop). Very great power savings are possible, although the wakeup time can increase past ten seconds.
  • S4: Hibernation. The system state (including RAM contents) are saved to non-volatile storage (usually the hard disk) and everything is powered down. This state saves the most power, but waking takes longer if you have a lot of RAM.
• G2: Soft off. The OS is shut down and the system is powered down. Most modern desktop PCs enter G2 when the user tells them to "Shut Down". Some events can cause the system to restart; the most common event is the user pressing the Power button.
• G3: Mechanical off. The OS is shut down and the system is disconnected from the power supply (usually by a switch on the back of the PSU). Upon reconnection, the system enters G2 without further intervention.