If you look at the description of many modules, you will see this message or something similar:
Penalty: Using more than one type of this module or similar modules that affect the same attribute on the ship will be penalized.It sounds ominous, but all it means is "the more of these you use, the less effective they will be". The above rule of thumb is simply boiling this down to the most popular cutoff point.
Back in the day, things didn't use to be this way. All modules applied their full effect, all the time. This meant that some ships which had very high numbers of mid or low slots, could fit multiplier modules in all of them, resulting in insane total modification. We're talking "insane" as in "battleships moving faster than frigates by filling their low slots with nanos" levels of insane. It was so crazy that it started breaking the laws of physics.
So it was changed, and the Machariel shed a manly tear.
How big are the penalties?
It's not a linear progression, and the penalty grows the more modules there are. The actual equation and visual representation of how effective your modules are is below:
Note that 100% simply means 100% of the module's effect. For example, a Gyrostabilizer II increases projectile damage by 10%. The second gyro would increase the damage by a further 86.91% * 10% = 8.7%. And so on...
Summing up these modifiers, this is how much you get out of fitting a number of identical modules:
Update: Turns out I'm a big fat wrong liar, and there is no such thing as additive stacking. The graphs have been replaced with graphs showing the real multiplicative effect.
Math is fun, isn't it? Note how fitting the fourth module is only of tiny benefit, and all further modules are nearly useless... hence the rule of thumb.
In case you really love math and would like to play around with the stacking penalty calculation, you can use this function for the total multiplier (M) of n modules each giving a m bonus:
Does this affect guns? If so, it's stupid.
It would indeed be stupid if it affected guns, but fortunately, it does not. "Absolute value" modules such as guns, shield extenders, armor plates, etc. are exempt from stacking penalty.
Are there other exceptions?
Yes. Here are some notable ones:
- Damage control modules. The boost they give to shield and armor resistances does not count as part of other resistance-modifying effects.
- Fitting modules. Co-processors, reactor control units, capacitor rechargers, cargohold expanders, and other such modules do not suffer stacking penalty.
- Trimark rigs and CDFE rigs. The amount of extra armor or shield hit points you receive from fitting multiple of these rigs does not diminish with each successive rig. This makes them extraordinarily powerful and popular on ships that need a lot of buffer tank (such as battleships or capital ships).
- Implants. Bonuses from implants do not count towards the stacking penalty of a certain attribute. Snake implants are an exception, to avoid excessive speed problems.
There are a few others, but I'm not going to attempt to go hunt down all of them. With very few exceptions (such as falloff "ambit" rigs), all stacking-penalized modules are indicated as such in their description. A quick search should let you know if a module is stacking penalized, even if it does not have the warning in its description.
What if I'm using modules of different strengths? Is it possible that the strongest one gets penalized for weaker ones "counting before it"?
No, not at all. Effects are "counted" in order of their strength.
Suppose I had an Energized Adaptive Nano Membrane II fit (20% resistance bonus to all armor resistances) along with an Armor Explosive Hardener II (55% bonus to explosive armor resistance). Since the hardener is the highest modifier, it gets counted first, and thus receives the full 100% of its effect, or 55%, while the EANM's explosive component gets counted second, receiving only 20% * 86.91% = 17.38% of its effect Since the other resistance bonuses the EANM provides do not conflict with anything else, they receive their full effects of 20% each.
My head hurts. Can I take advantage of all this somehow, or is this just hot air?
You can! By just knowing and applying all this, you already have an advantage over many other players. Some don't know better until it's too late (as one thing wrong with that fit).
Now go forth and conquer!