[Ward]

To that formula:

This is a formula that counts with: Space reqired by component's, damage dealt, different % to hit and of course overall defense points. It works also with many extreme examples.

Abbreviations used in examples and formula:

su = Space used. Total of space the components take up on ship.
sr = Space required for components. Count in kt. Add ALL components.
dd = Damage dealt. Pick any number bigger than 1.
mp = Modified percent. Attacker's percent to hit(again, choose any number) minus your defense adjustment(count OLNY bonus from component).
dv = Defense value. Add damage resistance(in kt) and additional points(shield or armor).

How it works?

1: (dd*mp1/100) - (dv1*sr2/sr1)
Count.

2: (dd*mp2/100) - dv2
Count.

Compare. Smaller wins.

Example1:

Let's say you have 50 kt of free space on ship. Enemy can deal you 1000 points of damage and he hits 60% of time. First ship has an armor and second a shield generator.

Armor requires 50 kt's adds 150 armor points and 15% to defend.
Shield generator reqires 40 kt and adds 375 shield points.

sr1 = 50
sr2 = 40
dd = 1000
mp1 = 45
mp2 = 60
dv1 = 200
dv2 = 415

1: 1000*45/100 - 200*40/50 = 290
2: 1000*60/100 - 415 = 185

Shield wins.

Example2:

Let's say you have 210 kt of free space on ship. Enemy can deal you 1000 points of damage and he hits 60% of time. First ship has 4 shield generators and 1 armor and second has 5 shield generators.

Armor requires 50 kt's adds 150 armor points and 15% to defend.
Shield generator reqires 40 kt and adds 375 shield points.

sr1 = 210
sr2 = 200
dd = 1000
mp1 = 45
mp2 = 60
dv1 = 1850
dv2 = 2075

1: 1000*45/100 - 1850*200/210 = -1312
2: 1000*60/100 - 2075 -1475
Shield wins(but slightly).

This is not a failure. Here really the shield wins. After all, if the armor gives you 15% to defense it is as if it gave you one sixth more defense. That also means that on average only after 6 shields it pays off to have an armor.
Try to slightly increase the % of shield. You'll see the results you probably want to see.

Of course this formula does NOT include all possibilities but it covers most common situations.