In Dec. of 2010 I decided to run a little experiment to see how much electricity my cell phone charger used when it was plugged in but not charging the phone. My original plan was to find out how much of the fabled 'vampire electricity' my phone charger used. I plugged in the charger into a kill-a-watt meter which measures electric usage. I left the charger plugged in over night and it had used 0.00 kWh of power. But then I had to charge my phone. I ended up plugging the phone into the charger and measuring how much power it takes to charge my phone as well as keep the charger plugged in 24/7 when the phone is not charging. I figured I'd just leave the meter plugged in and get a longer term measure of how much electricity my charger uses.
What I measured: I know for a fact that I charged my own phone and its mic. But my wife also used the same plug in a little. Altogether things that I measured are : Charging my cell phone, charging my bluetooth ear mic, charging my wife's phone some of the time, electric use from the chargers when plugged in and not charging, and maybe a little usage of the paper shredder that is near the charger in question.
This is not very scientific.
I have two pictures that show the kill-a-watt meter in use. The first picture is from Dec. 20th and the second is from Jan. 2nd. However the chargers were removed for about 4 days around Christmas when we were visiting family. In total this then measures about 8 days of usage. The first picture shows 0.03 and the second shows 0.06 so the difference is 0.03. In the first 8 days the usage was 0.03 kWh.
Note I had to brighten the pictures a little so you could see the read out. There isn't good lighting in the location where the meter is plugged in.
Dec 20th showing 0.03 :
On Jan 14 the meter was still plugged in and it read 0.12 kWh total. So in another 12 days there was another 0.06 in usage.
In 20 days total the usage is 0.09 kWh. So an average day uses 0.0045 kWh.
At that rate over a year the usage would average about 1.6425 kWh.
Electricity averages around 10-11¢ per kWh. So the cost to charge 1 phone and have the phone charger plugged in all year round is in the ballpark of 16-18¢ for a year. But there were other things plugged in some of the time like my wife's phone and possibly even the paper shredder.
I'm going to make a ballpark estimate that it is costing me about 1¢ per month to charge my cell phone.