This is something that's mentioned here or there: Power Backpacks. (Or, similar terms.) Devices which feed power to weapons such as a Power Fist or a Multi-Laser. Still, I can't find anywhere that actually gives the capacity, weight, or cost of any of this.

Additionally, could a single power backpack be wired to provide power to a Multi-Laser as well as a Power Fist, or would you be stuck with carting around an extra backpack to switch between?

And as far as Clip Size goes, does a Backpack only provide one clip? Do you have to charge it, or can it be replaced?

They usually have a rule covering ammunition weight, iirc it is 10% of the weapon's weight.

You can use rules from Ascension. P.145 list Backpack Ammo/Power Pack - 15 kg.