A Japanese appliance is plugged into a transformer so that it can be used in the US. This steps down the voltage from 120 V to 100 V (?). I’m wondering about the transformer’s power use when nothing is plugged into it, but it’s plugged into the wall. It remains warm. Is there any way to estimate its power usage, or is this an “it depends” situation, thus necessitating the use of some sort of meter?
Depends on the transformer but in general, the larger the core the greater the unloaded power draw. Easiest way to measure it is to plug in to a Kill-A-Watt power monitor or similar device. If you want a guesstimate, figure a loss of about 1-2% of the rated load. So, if the transformer is rated for 1000 VA, you’re losing about 10-20 VA with no load, or about 80-160 mA of current.