The title pretty much sums it up... Hyperbole aside, how much actual power is consumed by a typical wall wart when the device it's attached to isn't powered on? And are there designs for wall warts that minimize or eliminate this sort of parasitic load?
Answer
About 1 billion percent. No useful work is being done except to heat your house, and energy is being dissipated which would not be dissipated if the wall wart was not plugged in. The actual quantity is small, so I wouldn't worry about it too much if you're concerned about your electricity bill - A few cell phone chargers left in the wall 24-7 won't make a dent. For some math, if a device is just barely CEC certified (0.5W no-load dissipation) and left in the wall unused all month, you have 730 hours (in one month) * 0.5W * 0.001 W/kW = 0.365kWh [kilowatt hours] and electricity costs of, say, $0.20/kWh, and you have a cost of $0.07 Find a dime, pay the bill. Compare that to, say, your 73W fridge, which will be around $10/month.
If, however, you have a lot of wall warts, or the small losses are significant to you, look at switch-mode power supplies. They're more expensive, but more efficient both at no-load conditions and during use, and switch-mode supplies will actually regulate, as opposed to transformer-based designs which require further regulation circuitry. You can tell what kind a converter is just by the weight - A switch-mode supply will have just a lightweight circuit board with some electronics inside, while a traditional wall wart has a big, heavy transformer. I've used CUI switch-mode PSUs before, they're a good company with a nice line-up: See this page for some of their AC-DC converter products. See their Compliance page for more information on efficiency ratings.
Note: Some people use the term "wall-wart" for the transformer-based devices only, while others use it for both switch-mode and transformer-based wall plug-in AC-DC converters.
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