Save energy with Windows power management

At the risk of stating the blindingly obvious, your computer needs energy to run.

But with the unpredictable costs of this energy and increasing concerns over the long-term sustainability of power generation, there are great incentives to try to reduce your computer’s power consumption to the absolute minimum.

This feature aims to give you some practical advice on how to make effective use of the power management capabilities of your PC, as well as giving you some more in-depth technical background information to help you understand how it all works.

We’ll look mainly at Windows XP and Vista in this feature, although we will mention earlier Windows versions and Dos where relevant.

Luckily, setting up a good Windows power management scheme isn’t difficult and doesn’t need any advanced hacking skills – you don’t even need to edit the Registry. And for the really lazy we’ll point you to a free tool that can do it all for you. But for those who prefer the hands-on approach, all it requires is that you understand what you’re doing – which is where this feature comes in.

ACPI
The ancestor of modern PC power management was Advanced Power Management or APM (see 'APM – a bit of prehistory' below). This was a brave – if ultimately doomed – attempt to help PCs save power, but it eventually led to its successor, the Advanced Configuration and Power Interface (ACPI), which first appeared in Windows 98.

ACPI handed responsibility for power management to the operating system (although as we’ll explain later some Bios settings can still affect the way Windows works). This concept is known as operating system-directed power management, or OSPM.

In some Bioses you may see various ACPI settings, including a setting to enable or disable ACPI. This is to support non-ACPI-aware operating systems, for which you can go back to using APM at your peril. But note that if your Bios has such a setting, you should never change it after Windows has been installed – doing so could cause Windows to refuse to load. If the setting is wrong, the only safe time to change it is before doing a fresh reinstall of Windows.

The reason for this is that Windows installs a different hardware abstraction layer (Hal) for each type of PC hardware. The Hal is the software component that allows Windows to run on a vast range of different PCs: the Hal talks directly to the hardware, and Windows talks only to the hardware via this intermediate layer.

During a fresh installation, Windows tries to determine what kind of Bios your PC has and chooses the Hal accordingly. There’s more technical information on this process at the Microsoft Knowledgebase, in article 314088. Once installed, changing the type of Hal requires a fresh install of Windows, which is why you shouldn’t change the ACPI status in your Bios.

But assuming your PC is ACPI-compliant and Windows has correctly determined this (which isn’t a certainty, if the Bios is badly written or contains bugs), your way is clear to start optimising your PC’s power management. Let’s start by jumping straight into the thick of it by looking at one of the most confusing terms in power management – sleep.

Sleepytime blues
Booting up a PC can take what seems like an eternity, especially when you’re in a hurry. This is because your PC’s system memory doesn’t save information when it’s powered off – it’s what’s known as volatile memory. So, the operating system has to start from scratch every time, loading drivers, data and programs. Electronic components also have to be powered and initialised every time you turn the PC on, which adds to boot time.

Manufacturers realised that it would be much better for users if the PC could run in a reduced power state, ready for instant wake-up such as a TV or other consumer electronic device. This is known as sleep or standby mode, although there’s no real standard definition of these terms.

ACPI offers system designers two main ways of making a PC sleep. The least effective way is called Power-on-suspend, or Pos. In the parlance of ACPI it’s called the S1 sleeping state (see the attached PDF table, ACPI sleep states, for a list of these power states).