AC power supplies: keep them from zapping your wallet and the environment.(CR Technology)(alternating Current )

You may not think much about power supplies, those power cords with a bricklike appendage that converts AC power into the DC needed by cell phones, laptops, and a host of other devices. But touch that brick while any of those devices is on and it will probably feel warm. That's the energy lost during the conversion process. The power supplies hidden within desktop computers, TVs, cable boxes, and other appliances also waste energy.

With typical use, AC power supplies can waste $20 to $50 of what you spend annually on electricity. Nationwide, power supplies waste more than 58 billion kilowatt-hours yearly equal to the annual output of 10 large power plants. That extra energy output translates into 40 million tons of the greenhouse gas carbon dioxide released into the atmosphere each year, according to Ecos Consulting, an environmental consulting firm.

A major culprit in this waste is the type of adapter known as a linear power supply or transformer, which typically has an energy-efficiency rating of 30 percent to 60 percent. That means it loses 40 percent to 70 percent of the energy converted to DC when powering an appliance. A transformer can consume 2 to 5 watts just by being plugged in. Manufacturers like them because they're inexpensive to make.

More-efficient designs, called switching power supplies, can offer up to 90 percent efficiency They use far less power, even when not powering an appliance. They also cost slightly more to produce.

As of Jan. 1, the Environmental Protection Agency is expanding its Energy Star program to include external power supplies. Initially, to qualify for the seal, a power supply's average efficiency must fall in the top 25 percent of units on the market.

Energy Star-rated external power supplies should start reaching consumers during the first half of 2005. The Energy Star logo will be on the product box, but not on the power pack itself. That could mislead consumers about whether the rating applies to both the power supply and the device with which it comes. But Andrew Fanara, team leader of the EPA's Energy Star program, says that the language on the box will be clear.

Although most power supplies in use are internal, the Energy Star program won't cover those until at least the end of 2006 because they're more complex, Fanara says.