You have some other more demanding problems if the power usage of an additional DSL modem is a concern for you.

Well okay, you shelled out $29.99 for a new shiny USB DSL modem. How many years should pass to at least have a ROI?

Half?

A device runs on electricity and performs a function. It consumes power at a rate of x.

You're telling me that a device that performs the same function will consume power at a rate of precisely x/2 simply by virtue of being plugged into USB?

How much power difference?