The Backpack itself is too heavy, and from there the numbers go all wrong. Its one thing if it was slightly less efficient, but Lascannons are pretty much the only weapon where that backpack is advantageous.
The entire point of it is the efficiency of bulk purchases either in ammunition carried, and/or action economy. You HAVE to make up for most of the weight with ammunition, without forgetting [or giving nearly as much weight to as its been so far] the improvements caused by not having to reload for far longer.
Hellpistol/gun Capacitor: 80 shots, 5kg
Heavy Capacitor: 250 shots, 15kg
-Here, its a bit less efficient, but you're going 80 hell weapon shots without reloads, whereas using clips at quarter capacity would require reloading seven times for 4kg. The capacitors are too large to just plug into the gun, but they're not a rucksack. The small ones are about canteen-sized, usually affixed horizontally to the lower back, while the larger capacitors are an obvious thing, as well as a significant amount of weight.
-Some may carry a second small capacitor on occasion, but multiple heavy capacitors are rarely assigned to a trooper under most circumstances.
From here, an actual backpack rule for other weapons
Backpacks function for any clip or magazine-based weapon not stated as being fed externally in the first place. While smaller models only reduce loading times, the basic carrying frame is standardized, and efficiency improves with larger units:
Small: 5kg. Carries the equivalent of four times the normal clip capacity of a weapon.
Standard: 10kg: Holds ten times the weapon's normal ammunition capacity.
Heavy: 20kg: Holds 20 times the weapon's normal ammunition capacity in seperate packs, chambers or cells: These packs are generally used as a squad's supply rather than personal unit, and allow rapid distribution: While capable of discharging all available ammunition through the wearer's weapon, Standard clips or magazines may be replenished from the pack's capacity using a number of actions equal to the normal reload time of the weapon [rapid reload may apply].