When you fit multiple modules of the same type to your ship, the in-game information for many of these modules reports:
Penalty: Using more than one type of this module or similar modules that affect the same attribute on the ship will be penalized.That raises the question: what are the stacking penalties for fitting multiple modules of the same or similar type? They are:
- The first module is 100% effective;
- the second is 86.9% effective;
- the third is 57.1% effective;
- the fourth is 28.2% effective;
- the fifth is 10.6% effective; and,
- the sixth module is 3.0% effective.
The effect on "similar types" happens when you fit several modules that all affect the same attributes of your ship. For instance, two Energized Adaptive Nano Membranes will have stacking penalties, as will the active or passive hardeners for specific types of damage that you fit to the same ship. In each case, it will be the module that is most effective at each step in the math that will be penalized the least.
To check whether a mod has this stacking penalty, check the mod description (or see the bottom of this post). But most of the time, stacking penalties apply to modules that have an additive or subtractive percentage impact either on your own ship or another ship. Modules that have an integer effect on your own or another ship usually do not have stacking penalties. For instance, Stasis Webifiers (which reduce ship speed by a percentage) have stacking penalties, but Warp Disruptors (which have reduce warp strength by an integer value) do not. Most often, stacking penalties are most often thought-of when it comes to defensive modules (notably the Adaptive Invulnerability Field and the Energized Adaptive Nano Membrane) and weapon upgrade modules (the Heat Sink, Gyrostabilizer, Magnetic Field Stabilizer, and Ballistic Control System).
Stacking penalties can sometimes have surprising results as well. For instance, the effect of five small stasis webifying drones, the Warrior SW-300, stack. So the first SW-300 reduces target speed by 5%. But the fifth one reduces target speed by only 0.53%. So while intuitively you might believe five small webifying drones would reduce target speed by 25%, in reality they reduce it by only 14%. It is therefore more effective on paper to launch a single large stasis webifying drone at a target (for a 20% reduction in speed) than five small drones with the same size and bandwidth.
Though of course the smaller drones being half again as fast might actually catch the target whereas the large one might not...
Finally, it's worth noting that there are two modules in the game that do not have stacking penalties with other similar modules: the Damage Control and the Reactive Armor Hardener. But of course you can only fit one of these per ship. But if you fit both of them, then the less effective one of these two modules will be stacking penalized!
You can read more detail on this topic including the actual :math: formula for stacking penalties and a table showing which module types are stacking penalized and which are not at this excellent article on the EVE University Wiki.
Occasionally on Sundays, I will be defining a common EVE term for those who might not have heard it. If you have a suggestion for such a term, please drop it into the comments.