Think of it this way:
- Your portable device uses energy. Since recharging batteries is not 100% efficient (among other things, heat is a byproduct), it takes a bit more power than an otherwise equivalent non-battery-powered device does.
- That charger may give off perceptible heat even when nothing is plugged into it. In the summer, this may represent a double-use of electricity, once to generate the heat, and once to remove it via air conditioning.
As an aside, I like the little Kindle charger as a generally-useful tool:
- Its output is 4.9V at 850mA. My USB Sanyo NiMH charger NC-MDU01 specs say it draws 500mA at 5V. I can charge NiMH with this plugged into the Kindle charger.
- It's 100-240V 50-60 Hz which makes it suitable for international travel.