If you come across enemies (or even BoS neutrals) wearing power armor, sneak behind and vats the fusion core, if you do it right they'll have to eject it and you can salvage their pa frame (although it will be marked as stolen). If you kill an enemy (or BoS) wearing pa (dead lying on the ground) and they still have armor pieces equipped shoot the pieces until they disappear (zero health), they will then have zero weight.
Companions can use pa without fusion core.
