When appliances are off, they usually consume power at very low levels of between 1 and 10 watts. To measure such small loads, it is important to have the proper equipment. Most watt-meters have uncertainties roughly equal to the power use of a typical appliance in standby. In addition, the power supply and other electronic components introduces harmonics and other distortions. The result can be very poor power factors (sometimes below 0.25), which may contribute to misleading measurements. At LBNL, we use the EPD PLM1-LP, a watt meter that was custom built for measuring power at very low levels.
If you are looking for a meter capable of accurately measuring standby power consumption, the links page has a list of companies that we've found carry such meters.