Any electronic product that plugs into a wall outlet requires an AC-DC power supply to convert the high-voltage AC power provided by the electric utility into low-voltage DC power. Power supplies are embedded in some products, such as appliances and desktop PCs, while products such as mobile phones and broadband modems use external chargers or adapters.
Many power supplies use outdated technologies that waste a substantial amount of electricity. For instance, copper-and-iron linear transformers—a century-old technology still widely in use today—operate at efficiencies as low as 25 percent, meaning that for every watt of power supplied to the end product another three watts are wasted as heat.
Another problem is that efficiency typically declines as the load on the power supply decreases, such as when the end product enters "standby" mode. Thus the small amount of electricity needed to power the remote-control sensor of a TV or the clock on a microwave oven is delivered very inefficiently. Some devices consume power for no purpose at all, such as a charger left plugged into a wall outlet, a condition called “no-load”. (These devices are known as “energy vampires” because they have two “teeth” and suck power out of the wall continuously.)
The average home contains dozens of devices that spend the vast majority of their time in standby or no-load conditions, so the waste adds up. A 2008 study found that 13% of the electricity used in the average California home is for products in standby or other "low-power" modes.