I remember times when laspistols used fusion packs as ammo. Maybe it was not fun to look for it, but it was seeming logical for hi-tech weapon to use hi-tech ammo. Now it's just weird to imagine hi-tech laser weapon to use 1/10 of regular everyday battery to fry someone with high-intensity laser beam. Also, why would anybody design a secondary weapon that you need to carry attached o rather bulky and heavy box? Will you carry a pistol with cable protruding from it to UPS that you need to carry around? And what about powerful laser rifle for troops, that uses AA batteries in magical box? It just doesn't seem truth.
The idea is simple: give laser weapons plutonium batteries as ammo. They are not so scarce like hydrogen, and from design side it's better to give a weapon an autonomous power supply (like microreactor with 1 plutonium cell for pistols and 5 for rifles), than having it attached with cable to something. Maybe there could be a mod to make it use advanced UPS again, but not the regular one: I know, its middle of 21st century, but firing laser beam and 5 min use of flashlight for a same amount of energy, really?