PC power management
PC power management refers to software-based mechanisms 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 uses about 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 can be due to PCs and monitors.

While most PCs allow low power settings, there are frequently 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. Operational testing has shown that on any given day an average of over 50% of an organisation's computers will fail to go to sleep, and over long periods of  time this affects over 90% of machines. This leads to most computers having the option of customising power management systems, and has created a market for third-party power management software to further control a computer’s power use.

Windows 'Insomnia' (Sleepless PCs) 
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:

 Legacy or non-power management aware applications
 Open file handles on remote computers
 Faulty mice which can cause pointer drift. This makes the operating system believe that a user is present
 Scheduled maintenance tasks causing significant CPU activity
 High network activity

Software solutions 
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 macOS 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.

There is a significant market in third-party PC power management software offering features beyond those present in the Windows operating system.  Notable vendors include 1E NightWatchman, Data Synergy PowerMAN (Software), Faronics Power Save, Verdiem SURVEYOR. and EnviProt Auto Shutdown Manager

Some studies have suggested that power management tools can save on average 200 kg of CO2 emissions per PC per year, and generate $36 per PC per year in energy savings. This makes power management software an attractive option for large companies.

See also
 IT energy management
 ACPI

References 

Personal computers
Sustainable technologies
Computers and the environment
Power management