Power priority can be set for most modules that drawpower. Its purpose is to control module shut down order in case the power plant's power output is not enough to satisfy all modules. This is commonly due to the weapons deploying (which allows for shutdown of some modules deemed unimportant in combat, such as cargo hatch, fuel scoop, FSD interdictor, GFSD Booster, AFMU, etc.).
When power output can not meet the requirements, whole module groups are shut down, according to their priority, starting with priority 5 first and 1 last, until few enough modules remain to be functional at current output.
Damage to the powerplant causes reduced power output:
Power Plant Malfunctioning (Integrity < 80% and > 0%, at random): 40% Output
Power Plant Destroyed (Integrity 0%): 50% Output
Power Plant Destroyed & Malfunctioning (Integrity 0%, just destroyed or random): 20% Output
If a modules draws too much power push it further down the stack. Combine groups proposed here in accordance to the reduced output levels
Defensive utilities (Chaff, Heatsink)
Thrust capability (Thrusters)
Escape opportunity (Frameshift Drive)
Shield rebuilding (Shield Generator)
Anything else (Weapons, Shield Boosters, SCBs, etc.)
Non-combat relevant modules (FSD Interdictor, etc.)