Is it truly overpowered? I'm not sure. I feel that it is VERY dependent on what level it is, as well as what weapon is being used in combination with it. It's very easy to get multikills with it, and it may very well be one of the 'best' nano modules for the armor slot.
Energy Transfer I: A small portion of energy is recovered after each kill
Energy Transfer II: A greater portion of energy is recovered after each kill
Energy Transfer III: After a kill, the energy is restored fully.
This means almost an unlimited constant supply of energy (as long as the aforementioned requirements are met).
P.S. It's okay if you change your mind. This thread is solely for the purpose of gathering information and drawing conclusions.
So: is it unbalanced?