There is an anecdote about this:
A
KGB (FIS) officer calls some citizen who has been under investigation for a while, asking him:
— Excuse me, you don't know me, but I really need to talk to you on some very important matter; could you spend a few minutes?
— Ah! Well, hello, KGB officer!
— How can you know that I am a KGB officer?
— Oh, simply. You managed to reach me on the mobile phone which was
turned off (!).
As to the energy needed to power your device, these days it's not a problem anymore, as you can always get power over IP: please see my April's CodeProject article:
Power Over IP: Testing of the First Experimental Facility[
^] :-).
Good luck,
—SA