The S3 advantage
With newer computers come higher power bills. Modern processors, RAM chips, and hard drives consume much more power than their predecessors did, and the trend is likely to continue. While hardware manufacturers continue to work at reducing the levels of power consumption, the reduction is usually just a reduction in the leap from the previous generation, as opposed to a net loss. However, you can reduce the amount of power used by the PC when you are not there, and on some PCs, you can even change how much power is used when you're using it.
This is where the S3 sleep state comes in. In older power-saving modes (S1 sleep state), the computer could shut down some components, such as the hard drives and the monitors, after a period of time to reduce power usage. However, the CPU, fans, and other internal components continued to run at full speed. With the S3 power management, your computer is nearly out cold. It maintains a minimum amount of alertness, just enough to wait for you to wake it up.
Unlike older "hibernate" schemes, the computer is not actually off with its state saved, it is still on. When you wake it up, you don't need to wait for the BIOS to post or the operating system to come up enough to restore the saved state. Instead, your computer takes only a few seconds to be ready for work, right where you left off. During this deep sleep, the computer will typically draw about 2 to 3 watts of power -- a far cry from the small reduction in power usage with the older S1 sleep state.
Using S3 sleep state in Windows XP
S3 sleep state is enabled by default in Windows Vista. In Windows XP, though, a Registry key change is needed. TechRepublic offers a download that will change this Registry setting for you. This gallery looks at Windows Vista, but it's applicable to Windows XP as well. The Power settings section of the Control Panel is a bit different, but the rest of the settings should be the same.