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Power Savings: 80 PLUS, Energy Star, Advanced Power Management

Power Supply Reference: Consumption, Savings, And More

Over the past few years, energy conservation has become a major focus for appliances, electronics, and especially for computers. Saving power can be accomplished through several means. One is to use more efficient components that simply use (or waste) less energy to do the job. The other is to properly manage the computer hardware so that components that are not being used are powered down or put into standby modes. By using more efficient components and turning off specific components of the PC when they are not in use, you can reduce the electric bill and avoid having to power the computer up and down manually. This not only saves energy, but it makes the system more convenient to use. The following sections discuss both of these approaches to saving power.


When it comes to computers, one of the major factors in overall energy consumption is the efficiency of the power supply unit. In 2004, the Northwest Energy Efficiency Alliance (NEEA) funded the 80 PLUS Program to encourage computer manufacturers to improve the energy efficiency of their machines by installing highly efficient power supplies. Ecos Consulting, who manages the program, tests and certifies power supplies as being 80% (or higher) in efficiency. To help offset the cost of producing more efficient designs, the program also pays incentives to manufacturers producing PSUs and systems that are certified.

Systems with more efficient power supplies consume on average from 15% to 30% less power than conventional designs. This can result in a significant energy and cost savings over the life of a system. In addition, the resulting lower heat output both improves system reliability and saves additional energy in cooling the system as well as the surrounding environment.

The 80 PLUS program currently has five levels of certification, from 80 PLUS to 80 PLUS Platinum. Each level of certification signifies different minimum levels of efficiency, which are measured at three different loads (20%, 50%, and 100%). The following table shows the details of each of the certification levels.

80 PLUS Certification Levels
80 PLUS RatingEfficiency at 20% Rated LoadEfficiency at 50% Rated LoadEfficiency at 100% Rated Load
80 PLUS80%80%80%
80 PLUS Bronze
80 PLUS Silver85%
80 PLUS Gold87%
80 PLUS Platinum90%

How is this efficiency determined, and what is the overall effect? The PSU in a PC converts the high voltage (120 V in the USA) AC wall current to 12 V and lower DC voltages for use in the PC. Unfortunately, no PSU is 100% efficient, meaning that some of the power is lost or used up during the conversion and ends up being dissipated as heat. Conventional PSUs are or were normally about 70% efficient, which means that 30% of the energy drawn from the wall socket is wasted and ends up as heat. As an example, let’s take a system that draws 250 watts total. The table below shows the resulting AC power draw and the amount of wasted energy if the PSU were 70%, 80%, or 90% efficient. 

The Effect of PSU Efficiency on AC Power Draw and Wasted Energy
PSU Classification 
80 PLUS80 PLUS Gold
DC Power Draw (W)250
AC Power Draw (W)
Wasted Energy (W)107

As you can see, when supplying the same 250 watts of power to the system, the actual amount of power used, and consequently the amount of energy wasted, varies considerably. A more efficient PSU can save a tremendous amount of energy and money over the life of a system. Because of this, I highly recommend 80 PLUS certified power supplies, especially those earning the higher efficiency ratings.

Energy Star

Energy Star is an international standard for energy-efficient consumer products, including computers and power supplies. The U.S. Environmental Protection Agency (EPA) introduced Energy Star as a voluntary labeling program designed to identify and promote energy-efficient products. The first products labeled in the program were computers and monitors. In the years since, Energy Star has become an international standard, and the label can be found on new homes, commercial and industrial buildings, appliances, office equipment, lighting, electronics, and more. Devices carrying the Energy Star logo generally use 20%–30% less energy than required by federal standards. In addition to Energy Star, many European-targeted products are labeled with TCO Certification, a combined energy usage and ergonomics rating from the Swedish Confederation of Professional Employees (TCO).

Starting in 2007, the Energy Star computer 4.0 specification required the use of a power supply that meets the 80 PLUS standard. In 2009 the 5.0 specification was released and now requires a power supply meeting the 80 Plus Bronze standard, for a minimum of 85% efficiency (at a 50% load).

Advanced Power Management

Advanced Power Management (APM) is a specification jointly developed by Intel and Microsoft that defines a series of interfaces between power management–capable hardware and a computer’s OS. When it is fully activated, APM can automatically switch a computer between five states, depending on the system’s current activity. Each state represents a further reduction in power use, accomplished by placing unused components into a low-power mode. The five system states are as follows:

  • Full On—The system is completely operational, with no power management occurring.
  • APM Enabled—The system is operational, with some devices being power managed. Unused devices can be powered down and the CPU clock slowed or stopped.
  • APM Standby—The system is not operational, with most devices in a low-power state. The CPU clock can be slowed or stopped, but operational parameters are retained in memory. When triggered by a specific user or system activity, the system can return to the APM Enabled state almost instantaneously.
  • APM Suspend—The system is not operational, with most devices unpowered. The CPU clock is stopped, and operational parameters are saved to disk for later restoration. When triggered by a wakeup event, the system returns to the APM Enabled state relatively slowly.
  • Off—The system is not operational. The power supply is off.

APM requires support from both hardware and software to function. In this chapter, you’ve already seen how ATX-style power supplies can be controlled by software commands using the Power_On signal and the six-pin optional power connector. Manufacturers are also integrating the same type of control features into other system components, such as motherboards, monitors, and disk drives.

OSs that support APM trigger power management events by monitoring the activities performed by the computer user and the applications running on the system. However, the OS does not directly address the power management capabilities of the hardware. All versions of Windows from 3.1 up include APM support.

A system can have many hardware devices and many software functions participating in APM functions, which makes communication difficult. To address this problem, both the OS and the hardware have an abstraction layer that facilitates communication between the various elements of the APM architecture.

The OS runs an APM driver that communicates with the various applications and software functions that trigger power management activities, while the system’s APM-capable hardware devices communicate with the system basic input/output system (BIOS). The APM driver and the BIOS communicate directly, completing the link between the OS and the hardware.

Thus, for APM to function, support for the standard must be built into the system’s individual hardware devices, the system BIOS, and the OS (which includes the APM driver). Without all these components, APM activities can’t occur.

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