Q: Many devices that are "always on" while seemingly "off" draw power so that they can spring into action on demand. How much electricity does a television, for example, use when plugged in but not turned on?

A: Lawrence Berkeley National Laboratory has done extensive studies of standby power since 1996 for the Department of Energy. In particularly inefficient appliances, standby power use can be as high as 20 watts.
"For a single appliance, this may not seem like much," the laboratory's web site says, "but when we add up the power use of the billions of appliances in the US, the power consumption of appliances that are not being used is substantial."

An exact reading of the standby power drawn by an individual appliance can be obtained only by using a fairly expensive energy meter or by turning off all the rest of a home's appliances and checking the utility meter.
For making an estimate, a laboratory web site - standby.lbl.gov/data.html - provides tables of the minimum, average and maximum power used by appliances that cannot be switched off completely without being unplugged. For television sets, the laboratory estimates a minimum power use of zero watts, an average of 5 watts and a maximum of 21.6 watts.

New York Times