power loss in battery chargers...USB Answered
a transformer, being a pair of electromagnets, such that it reduces the voltage of household power to whatever voltage your battery powered device needs, will cost you somethin if its plugged into the wall, even if yoour phone, laptop, etc isn't plugged in.
but it seems that these 5v usb chargers are too small to contain a transformer. I dont think they get warm when they are idly plugged in. I imagine they may just have a rectifier and zener diode. Is that the case? Do they waste power when left in an outlet?