BTW.:
I know your post was old but I was looking for the same info so as I had found out I thought I would answer it.
Did you (Claust) get a permit for flying commercially near built-up areas in DK? And if you did - how?
It is very well explained here:
http://www.wikihow.com/Calculate-Kinetic-Energy
So - for a normal phantom with gimbal and go-pro it would probably be something like:
Ex=(2Kg*(10M/s)²)/2 = 100J