If I use Nuzleaf's "Surprise Punch" and my opponent has no Benched Pokemon, what happens to the Energy on the Defending Pokemon?
If there are no Benched Pokemon you just do as much as you can. You do 20 damage but no energy is moved or discarded.
Source: Next Destinies FAQ (2012-02-09), TPCi Rules Team (2012-02-09)

Relevant cards

Note: All applicable cards may not be listed below.
BW4_71