Fitting Skills:
These are all the skills you need to fit all your desired modules onto your ship without running out of powergrid or CPU.

Engineering - 5% Bonus to ship's powergrid output per skill level.
Electronics - 5% Bonus to ship CPU output per skill level.
Energy Grid Upgrades - to fit Power Diagnostic Systems, which increase powergrid.
Electronics Upgrades - 5% reduction of sensor upgrade CPU needs per skill level, and to fit Co-Processors.
Shield Upgrades - 5% reduction in shield upgrade powergrid needs.
Weapon Upgrades - 5% reduction per skill level in the CPU needs of weapon turrets and launchers.
Advanced Weapon Upgrades - 2% reduction in powergrid needs of weapon turrets and launchers.

I believe the most common benchmark for PvP Fitting Skills is to have Engineering, Electronics and Weapon Upgrades all up to level 5, and Advanced Weapon Upgrades up to 3. The others can be at the level of your choice.
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Basic Capacitor Ski