PC power management

PC power management refers to the mechanism for controlling the power use of personal computer hardware. This is typically through the use of software that puts the hardware into the lowest power demand state available. It is an aspect of Green computing.

A typical office PC might use on the order of 90 watts when active (approximately 50 watts for the base unit, and 40 watts for a typical LCD screen); and three to four watts when ‘asleep’. Up to 10% of a modern office’s electricity demand might be due to PCs and monitors.

While some PCs allow low power settings, there are many situations, especially in a networked environment, where processes running on the computer will prevent the low power settings from taking effect. This can have a dramatic effect on energy use that is invisible to the user. The monitor may have gone into standby mode, and the PC may appear to be idle, but operational testing has shown that on any given day an average of over 50% of an organisation's computers would fail to go to sleep, and over time this happened to over 90% of the machines.

The Windows power management system is based upon an idle timer. If the computer is idle for longer than the preset timeout then the PC may be configured to sleep or hibernate. The user may configure the timeout using the Control Panel. Windows uses a combination of user activity and CPU activity to determine when the computer is idle.

Applications can temporarily inhibit this timer by using the SetThreadExecutionState API. There are legitimate reasons why this may be necessary such as burning a DVD or playing a video. However, in many cases applications can unnecessarily prevent power management from working. This is commonly known as Windows 'Insomnia' and can be a significant barrier to successfully implementing power management.

Common causes of 'insomnia' include:

Operating systems have built-in settings to control power use. Microsoft Windows supports predefined power plans and custom sleep and hibernation settings through a Control Panel Power Options applet. Apple's OS X includes idle and sleep configuration settings through the Energy Saver System Preferences applet. Likewise, Linux distributions include a variety of power management settings and tools.

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