Improving Efficiency in Power Supplies

November 05, 2007
November/December 2007
A version of this article appears in the November/December 2007 issue of Home Energy Magazine.
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Most office equipment and consumer electronic devices use external power supplies to convert high-voltage alternating current (AC) into the low-voltage direct current (DC) that they need to operate. The majority of these power supplies are far less efficient than they could be—their efficiency is on the order of 65%–70%. These power supplies consume about 2% of all electricity produced in the United States.

To encourage the adoption of energy-efficient power supplies, Energy Star developed a labeling protocol for energy-efficient external power supplies. The program was launched in early 2005 in the United States and China—the world’s two largest power supply markets. In addition, the California Energy Commission’s Public Interest Energy Research (PIER) program funded a two-year focused analysis of power supply energy savings opportunities. The project team—which also included the Electric Power Research Institute (EPRI), Ecos Consulting, and the Electricity Innovation Institute (E2I)—held a design competition to encourage efficiency improvements for next-generation power supplies; developed a standard method for measuring the efficiency of power supplies; and produced a design guide for efficient power supplies. This project resulted in four major accomplishments that should lead to widespread energy savings.

Improving Energy Efficiency

In 2005, the Energy Star program began labeling products—such as cell phones, PDAs, digital cameras, and camcorders—that are manufactured with Energy Star-qualified external power supplies. Eventually, more Energy Star products—including laptop computers, cordless phones, and office equipment—will incorporate the external power supply requirement. Qualifying power supplies will also be sold separately as replacement products. To qualify initially, a power supply’s average efficiency must fall in the top 25% of units on the market.

Another way the project partners encouraged research on advanced energy-efficient designs from power supply manufacturers was by holding an international design competition, sponsored by  the California Energy Commission and the Energy Star program. The winning designs were announced in early 2005. These new designs are more efficient, more compact, and in many cases dramatically smaller than typical power supplies on the market. They demonstrate the efficiency improvements likely to be made in products that will hit the market in the future, with gains of 5 to 20 percentage points above Energy Star specifications. Improvements continue to be dramatic, especially among low-wattage power supplies, which tend to be less efficient than higher-power units (see Figure 1).

To measure and compare power supply energy efficiency consistently and accurately, a standardized procedure is necessary. The project team developed such a procedure and used it to test hundreds of power supplies. The procedure is now available for use as a common technical foundation for power supply labeling, standards, and procurement programs. It has been adopted for use by the governments of the United States, China, Australia, Canada, Brazil, and several European countries.

The team has also published Designing AC-DC Power Supplies for Improved Energy Efficiency: A Technical Primer.  This guide identifies the main components and subsystems that contribute to the majority of losses in power supply efficiencies, and it recommends ways to improve designs.

The project team hopes that these four accomplishments will greatly reduce power supply energy consumption. The team estimates that if efficiencies of new external power supplies sold in the United States improved from about 30% to 80% in the active mode, annual energy savings would amount to more than 5 billion kilowatt-hours.

Applying the Technology

The technologies and procedures developed in this program can be applied to internal and external AC-to-DC power supplies for various types of electronic equipment, including digital displays, electronic timers, transmitters, receivers, DC motors or lighting, speakers, remote controls, keyboards, rechargeable batteries, and all AC-powered products that use integrated circuits.

The California Energy Commission has adopted mandatory energy efficiency standards, in active and no-load mode, for external power supplies for appliances sold or offered for sale in California since January 2007. Examples include external power supplies for laptops, cell phones, printers, scanners, personal data assistants, and digital cameras. All other external power supplies went into an effective compliance in July of 2007. The required efficiency levels are initially the same as those required by the Energy Star program and are scheduled to become more stringent on July 1, 2008.

In the near future, expect to see Energy Star finalize a labeling program for desktop computers that includes specifications for internal power supplies. Internal power supplies are located inside the devices that they power. This specification, which uses efficiency levels recommended by the project team, will go into effect soon.  Also, the International Electrotechnical Commission will consider adopting the standardized test procedures developed by the PIER program for international use.

Kristin Kamm, a senior research associate with E Source Technology Assessment Group, reports on lighting technologies, integrated circuits, advanced materials, and drive power.

This project was conducted by the California Energy Commission’s Public Interest Energy Research (PIER) program, which supports public-interest energy research and development that helps improve the quality of life in California and elsewhere.

For more information:

To learn more about efficient power supplies,  go to

For more on PIER, go to

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