The method to the madness of wall warts.

Blog post by Mary Kelley

So how is energy wasted from chargers that are left plugged in? It apparently depends on the charger. Chargers with iron cores attribute theirs to "eddie current" losses. Most chargers are transformers and in this case the magnetic field in the transformer causes magnetic domains in the iron core to flip back and forth creating heat. The rate at which the current is chopped is about 60/50 times a second. Switch mode power supplies apparently don't have this issue as much and therefore don't heat up. They usually have ferrite cores that result in less power loss thanks to a smaller core and they chop the incoming AC current at a much higher frequencies.

Typical iron core chargers waste about 4 watts when plugged in and are usually on the heavy side, when charging they use 6 watts so the waste is substantial. Typical losses from a switch mode power suppilies is about .2 watts so it's definately the way to go. If you aren't an electrical engineer (most of us aren't) then there's two ways you can tell if your charger is a power hog or not, if it's heavy and heats up chances are you got a hog, if it's light and cool to the touch in spite of being left on then it's likley conservative.