from france
well i said it p 130 ine 6 to 9. and page 135 last line for the weight of the power pack. so whe have power fits 13kg and the power pack 25 for a total of 38kg. power fist don't use amo granted but they require external energy source whitch is not inclunding in the weight of the power fist.
for the hellguns p118 it is a specific powerpack with a specific wheigth so it is not a good example plus it is still external (cables) as shown of pictures and minis. if you want a reduced weight you will have to use standart cell. but you will have far less shots. if you want more shots with more pucn you cant use a back pack powerpack and then you get again a 25kg problem.
.