Save energy with Windows power management

The S1 state reduces the power to S1-capable components, such as the processor, but the power supply remains on and fans may still be working. Components that don’t support S1 will be turned off. Memory is kept powered up.

On our ‘typical’ test system, entering the S1 state reduced power consumption from its working idle level of 150W to 200W to about 100W. All the fans remained on, but hard drives, graphics cards and other major components were off.

A much more aggressive sleep mode is the S3 sleeping state, or Suspend-to-Ram (STR). In this, system state is saved entirely to memory, meaning that just a small amount of power (from the 5V standby output of the power supply) is needed to keep the memory chips and other essential components operating.

On our test system, power consumption in this state dropped to just 15W, which is only a couple of watts more than the ‘shutdown’ state (13W) on our PC (this ‘off’ consumption figure is high due to the rather old and inefficient 600W power supply in our test PC. See our 'Switch on and save' feature for advice on buying an efficient model.

Hibernation
Both these sleep states above allow rapid wake-up of your PC – typically just a few seconds. But they still need a small amount of power and for laptops in particular this isn’t a good plan for long-term storage. If you leave a laptop in this state for a long time, eventually the batteries will run down and you’ll lose any unsaved data.

So a different approach is needed to allow zero power drain while still allowing a quicker startup than cold-booting. The answer is to store the system state data not in Ram but on the hard disk, a process known as hibernation, or the ACPI S4 sleep state.

When hibernation was first implemented, a separate hidden partition on the hard disk was often used. This wasn’t really a good plan – if a user deleted the partition, hibernation wouldn’t work.

So you’ll find most Windows PCs now use a special hidden system file, which is the same size as the amount of memory in your PC. The file is called hiberfil.sys and sits in the root directory of your system drive.

Writing the data stored in memory to the hard drive takes some time and it’s also a lot slower to read than Ram, so hibernation is noticeably slower than S1 or S3 sleep when shutting down or restarting your PC. The big advantage is that when the system’s hibernated, you can safely pull the plug or battery out of your PC and not lose any of the saved system state.

Hibernation first appeared in Windows 95, when it was called suspend-to-disk. It needed special drivers for each PC and so it wasn’t widely used. Windows 98 introduced ACPI support, but there were still many problems with hibernation, particularly on Fat32-formatted drives. It wasn’t really until Windows 2000 that it became reliable and didn’t need special drivers.

If the hibernation file is deleted for any reason it will be recreated when the system next hibernates. The file is deleted when you disable hibernation in the Windows Control Panel (see 'Magical wake-up' below).

The Vista difference
Vista radically changed the flexibility of power management on PCs, although many derided it as too complicated, with multiple ways of turning off the PC listed on the Start menu, including the new all-embracing Sleep mode.

Vista’s most significant new power-related feature is Hybrid Sleep mode, which is just a combination of the S3 and S4 sleep states. In Hybrid Sleep the system’s running state is stored both to memory and to the hard disk. This allows fast wake-up of a few seconds, but also provides a safety net if the power is turned off while the PC is asleep. If this happens, the data from the hard disk is used to resume the PC, just like hibernation.

More importantly, Vista is (usually) much more intelligent about how it sleeps, adjusting the type of sleep state, depending on what the PC is doing. For example, if you’re recording a video or downloading a file – if your hardware and software are fully ACPI-compliant – pressing the power button on the Start menu will turn the display off. When the tasks have finished, the system will gradually go into deeper sleep as determined by the timeouts in Control Panel, unless you wake it up from the keyboard or mouse, or another task starts.

Putting it all together
Now we know how it all works, it’s time to make use of our knowledge. First, you should make sure your Bios settings are correct. If you’re running Windows XP or Vista and your Bios is not set to enable ACPI, most power management functions won’t be available to you. But as we explained above you’ll need to reinstall the operating system.