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How much energy do computers consume?
Because computers consist of a wide variety of power-consuming components, power consumption varies substantially from one computer to another. To determine the size of an energy system, the key parameter that needs to be established is the average power consumption of the computer while running the types of software applications that will be used.

Computer processing units (CPUs), also known as processors or computer chips, are responsible for a significant percentage of a computer's power consumption. As the power of the processor has increased with each new generation of chips, maximum CPU power has risen as well. For example, the Intel P4 desktop processor, which is available at clock speeds ranging from 1.3 to 3.2 GHz, consumes significantly more power than the previous generation of desktop chips (Desktop CPU Thermal Design Power). Within a given family of processors, CPU power consumption generally increases along with clock speed. Processors designed to be used in notebook computers typically consume much less power than those designed for desktop computers.

While there is little published information about the power consumption of computers by the world's major manufacturers, there is even less information regarding "white box" computers assembled by local companies in developing countries. Many factors, such as the quality of power supplies and system design, can affect total PC power consumption. Locally assembled computers cost significantly less than imported models, and have come to dominate many local markets in terms of market share. In Guatemala, for example, white boxes account for 70% of all computers sold. Within a given country there may be dozens of local companies participating in the computer market. Local companies may well be willing to respond to request for information on the power consumption of their computer models.

Energy system designers often use a desktop computer's power rating to represent the electric load when sizing renewable energy systems. Information on the rating power is almost always available as it is printed on the computer's power supply for safety reasons. However, a variety of studies have shown that computers typically consume less than the maximum power rating. In some cases, computers have been found to consume as little as 5% to 50% of the rated power level (Ref) . One brand name computer manufacturer indicated that its desktop units are configured to consume no more than 75% of the power rating during typical operation (Ref).

Power Management

Computers reduce power consumption by shutting down system components when they are not in use. Power management enables a computer to enter reduced power states such as standby and sleep mode. Perhaps the best known power management guidelines are those set by the U.S. Environmental Protection Agency (EPA) Energy Star program. Desktop computers that meet EPA Energy Star guidelines power down to no more than 15 percent of maximum power use while in sleep mode. Computer models registered with EPA Energy Star typically consume between 5 and 25 W in sleep mode (Sleep Mode Power Consumption of Energy Star Qualifying Computers) . For the purpose of selecting ICT products, it is important to understand that Energy Star computer guidelines were designed only to reduce wasted energy while computers are idle, such as power consumed by office computers left on overnight. Computer products that are compliant with Energy Star guidelines are not necessarily energy efficient or lower power during normal operation.

There are a couple of power management standards for IBM-compatible PCs. Advanced Power Management (APM), first introduced in 1992, defines five power states: Ready, Standby, Suspended, Hibernation and Off. The successor to APM, Advanced Configuration and Power Interface (ACPI), has been available since 1997. Intel offers an emerging PC power management technology based on ACPI called "Instantly Available PC".

A computer's hardware, BIOS and operating system must all support power management in order for it to function properly. Older computer models do not necessarily have all these components, or the capabilities may exist but they are not enabled. If a PC is running Windows 98 but the BIOS does not support APM or ACPI, then power management will not function properly. All new computers running the latest Microsoft Windows or Apple Macintosh OS support power saving modes such as standby. Computers will older hardware and operating systems may support limited or no power management features.

Table: Operating Systems and Power Management
This table provides a quick summary of the extent to which common desktop operating systems support power management.

Power consumption using Linux can be expected to emerge as a topic of additional research over the next few years. A small but growing number of developing countries are choosing Linux in order to reduce licensing costs, particularly when using donated computers. In November 2003, the DabaweGNU Linux user group of Davao City, Mindanao and the Philippine Department of Education donated a server running Linux Terminal Server Program (LTSP) and a network of ten clients to the Kapitan Tomas Elementary school, with the intention of continuing to provide donated computers to schools in urban and rural areas. As organizations such as DabaweGNU extend their programs in off-grid areas and in areas where the grid is unreliable, power consumption will become more of a concern.

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