It might also do well to have the ship's power status tied to an actual "reactor" subsystem. Therefore if the subsystem is knocked out, you have a no-power situation. As a fighter you can run on internal batteries (as IPA proposed), until a support ship comes in to fix you up. Its possible that the battery is only sufficient to maintain life support and comm systems, so its like being disabled and disarmed with no shields. As for tables, we have seen ships in FS_retail that have a "reactor" subsystem, the Moloch, so a special subsystem flag would tell the engine to look tie the new power output to that subsystem. The use of a subsystem flag would also allow the moder to use a different name for the reactor subsystem, like "generator" or something. So, you have:
$Power_Output: 5.5 ;; the new one
and
$subsystem: Reactor, 5, 0.0
$flags: ("makes power") ;; or something like that
This would also allow a ship to have more than one reactor **cough**Lucifer**cough**. The amount of the total power that each reactor produces would be based on its subsystem strength. For exmple, if a ship has 3 reactors, with one having a subsystem strength of 6, and the other 2 having strength of 2, that means that the stronger reactor would be producing 60% of the ship's total power, and the other 2 would be producing 20% each. As these subsystems are dammaged or destroyed, the ship's total power available would drop.