Around the world, battery chargers for mobile phones, MP3 players and other useful devices are consuming electricity – even after the device itself has been unplugged. As long as the power supply or charger remains in the outlet, milliamps of current are flowing through them so that the electronics inside the power supply will have the right voltage ready when it’s needed.

Since that voltage is available instantaneously as soon as the power supply is plugged in, this is wasted energy.

A trickle of milliamps into a mobile phone charger doesn’t seem like much. But when it is multiplied by all the world’s chargers – the average person in many countries has four of them – billions of trickles combine into a torrent of wasted energy. The total amount of energy lost worldwide has been estimated as the equivalent of a 500 megawatt generating station based on 4 billion devices in service worldwide of which 25% (1 billion) are in service at any one time. If one‐half of a watt could be saved per device times 1 billion devices, it would amount of a savings of 500 MW. In the U.S., the ENERGY STAR® program, a joint effort of the U.S. Environmental Protection Agency and the U.S. Department of Energy, has targeted the wasteful ways of power supplies (sometimes known as wall warts). Over the past five years, it has established aggressive goals for increasing power supply efficiency. Figure 1 shows the efficiency requirements (vertical axis) of end‐use power supplies according to their capacity. The horizontal axis shows the allowable power consumption under no‐load conditions. A power supply must meet both criteria (i.e. fall into one of the EPS blocks) to earn an EnergyStar rating.