I would think the energy is drained from a internal reactor. Perhaps they get larger as the classes do.
Mini Nuclear - Fighter, ect ect...
However, I have always wanted for a Tactical class of weapons. I think of it this way. All the current weapons do is kill the shield, then hull. Say, however, You make a weapon do a far less amount of hull damage but it has pinpoint acuraccy enough to target devices and destroy them leaving the ship defenseless? I think this idea has been shot down before, because freelancer says you need to kill the shield first, then hull.