All the cell phone manuals I have ever seen recommend unplugging the transformer when not in use, in order to “save power”. But none of them says how much power. Can you help?
Several years ago I bought a Kill-A-Watt power meter and checked out a bunch of home appliances, including my cell phone charger. YMMV, by my charger consumed 3 watts when charging a phone, and 0.5 watts when no phone was connected.
Assuming the utility company charges a rate of ten cents per kilowatt-hour, if I leave my charger plugged in (but unused) for a month, it would cost me a grand total of four cents (or I would save that much if I bothered to unplug it).
There are a few true “energy vampires” out there, but most devices consume very little electricity in sleep mode. Still, there are occasional surprises. With my Kill-A-Watt, I discovered that my computer’s sound system (a Logitech system with four satellite speakers and a subwoofer with an integrated amp) sucks down 23 watts all the time. I never shut it off, so that costs me an extra $1.70 per month.
Any power brick that meets Energy Star requirements will have an idle power of 1 Watt or less. The newer standards are 1/2 Watt for standby power.
To put this into perspective, if there are 100 Million of these in use in the US, unplugging them all will save 438 GW Hours per year (if they are unused 12 hours/day.
Of course, on and individual basis 1W of wasted power isn’t very much. At 10¢ / KWH it would only cost 44 ¢ to let a 1W brick stay plugged in when not needed (12 hours/day).
As a rough estimate, my local power costs 1$/year/watt, so when I discovered (with my Kill-A-Watt) that an old MF printer I was using just to scan the occasional document was taking 10 watts (or 10$/year) even when turned off, I ditched it and invested in a modern (all-working) printer.
As has been posted though, unless you have an extraordinarily inefficient transformer, your cell phone gear isn’t worth thinking about. For less than 1$/year, it’s something I can safely ignore. 
If you’re interested in checking power consumption, there’s also the Belkin Conserve Insight and more that are fairly inexpensive. I replaced my printer, but almost as importantly didn’t replace my old washer and dryer when I found that they weren’t costing me anywhere near what I thought they were based on all those EnerGuide stickers.
<rant>
Can I just say since this is the Dope that words like “wattage” and “amperage” really annoy me? There are perfectly good words, “power” and “current” which describe those values that are measured in watts and amperes. Not everything electrical has to rhyme with “voltage”.
</rant>
EDIT: Just realized that “potential” is the proper term for “voltage” too. Does anybody know where this “-age” business started and why it seems to permeate the electrical world?
that doesn’t seem to be a huge amount of rantage.
There’s more than meets the eye.
“Potential” has many other meanings other than the electrical term, and I see “voltage” used far more often. That said, while I use “voltage” myself, I generally use “power” and “current” instead of “wattage” and “amperage”. Also, have you ever heard somebody use “overpotential” (which has a specific meaning, also note the text under the title) when they would usually say “overvoltage”?
Also, as far as standby power goes, older style AC transformer based adapters tend to draw more power when not loaded, compared to modern SMPS-based adapters, which are also much more efficient when loaded (the older style can be less than 50% efficient, SMPS designs are usually at least 70%, up to 90+% for modern designs).
Another thing to note is that some appliances, like computers, can draw significant amounts of power even when “off”; a modern computer has a standby output that is rated for at least several amps at 5 volts (have seen as much as 6 amps - 30 watts; of course, this isn’t the actual power draw), used to power the memory and some peripherals when it is in sleep mode (if there isn’t any need for this, you can go much lower; I myself make microprocessor-controlled circuits which use a similar power supply design, but the standby output is as low as a few milliamps, just enough to power an on/off control circuit, this is so it can turn itself off instead of using a manual switch; power draw in my newest designs is only around 20 milliwatts).
A couple more incorrect statements that drive me bonkers :
“The flow of current is 13 A.”
“Current is comprised of mobile electrons.”