Stacking penalties

Stacking penalties

Stacking penalties refer to an effectiveness reduction that is incurred when using two or more modules or rigs that affect the same attribute. Most, but not all, mods show in their discription if they are subject to this. Modules affected by stacking penalty multiplier include, but are not limited to: Electronic Warfare (target painters, webbers), resistance bonuses (hardeners, etc), Ship speed modules (nanofiber structure, inertia stabilizers and overdrive injectors), and others.

Note that it is the stat bonus that is affected, not the module itself.  For example, if you have a nanofiber internal structure (agility and velocity) and an overdrive injector system (velocity), the nano's velocity bonus would suffer a stacking penalty, but not its agility bonus.  Similarly with armour or shield resistance modules:  if you fit an EM hardener, an Explosive hardener and an omni-hardener (adaptive invulnerability field or adaptive nano plating); the EM/Exp resistances from the omni-hardener would be stacking-penalized but the Kin/Therm resistances would not.

Also of note is that module drawbacks (negative effects) can also be stacking penalized.

The Formula

The stacking penalty for module number n is

S(n) = 0.5^(((n-1) / 2.22292081) ^2)

The Numbers

1st mod: 100.0% effectiveness

2nd mod: 86.9% effectiveness

3rd mod: 57.1% effectiveness

4th mod: 28.3% effectiveness

5th mod: 10.6% effectiveness

6th mod: 3.0% effectiveness

As is clear, stacking more than 3 or 4 modules - unless you really have nothing else at all that you could fit there - that affect the same stat is fairly pointless, as your benefit is so tiny.

What suffers stacking penalties?

The first thing to note is that absolute effects are never stacking-penalized, only percentage effects. That is to say, things like +1 warp core strength, +1000 structure HP, +15m signature radius, +400 GJ capacitor are not stacking-penalized because they are not percentage (%) effects.

The second thing is that only modules and rigs are stacking-penalized. Skills, ship bonuses, implants, hardwirings and boosters are not. There is one exception to this, and that is the Snake pirate implant sets (the ones that affect velocity), which are stacking-penalized.

The third thing is that negative and positive effects are stacking-penalized separately.  So, one +% and one -% effect to an attribute suffers no penalties, whereas with two +% and two -% effects, one of each would only be 86.9% effective.  This situation doesn't generally come up in practice though, since if you're fitting modules that both add to and detract from the same stat on your ship, you're probably doing it wrong.

It can be hard to tell exactly what suffers stacking penalties and what does not.  For instance, the Overdrive Injector System II gives +12.5% velocity at a cost of -20% cargo capacity, and the description claims it suffers stacking penalties.  The velocity bonus is stacking-penalized, but the cargo drawback is actually not.  Similarly, the Warp Core Stabilizer II gives you one extra warp core strength at a cost of -40% targeting range and -40% scan resolution. It doesn't mention stacking penalty in the description, but the drawbacks are actually stacking-penalized.

We can learn from this that it is not the module itself which determines whether its effects are stacking-penalized, but rather the attribute affected.

Here's a handy table of what ship effects are stacking-penalized and which aren't. It has an important footnote and is not yet complete (and I'm still not 100% sure about some things). Remember, it is only percentage effects from modules/rigs that are penalized.

Ship attribute	Is it stacking-penalized?
Powergrid (including reduced-PG-need effects)	no
CPU (including reduced-CPU-need effects)	no
Cargo capacity	no
Capacitor capacity	no
Capacitor recharge rate	no
Shield recharge rate	no
Shield / armor / hull HP	no
Shield / armor / hull resistances	yes1
Shield boost / armor repair bonus	yes
Sensor strength (including ECM jam strength)	yes
Scan probe sensor strength	no
Scan resolution	yes
Targeting range	yes
Signature radius	yes
Velocity	yes
Inertia modifier (agility)	yes
Duration (cycle time) bonuses (except weapons)	no
Missile launcher rate of fire	yes
Missile damage	yes
Missile explosion velocity	no
Missile explosion radius	no
Missile flight time	yes
Missile velocity	yes
Turret rate of fire	yes
Turret damage modifier	yes
All optimal range (modules/turrets)	yes
All falloff (modules/turrets)	yes
Mining laser yield (including mining drone yield)	no
Drone damage	yes

¹ Damage Control modules and the Reactive Armor Hardener modules are stacking-penalized separately from all other resistance modules. That is to say, if you only fit one of them, it will always give it's full effect alongside one other resistance module that will also give it's full effect (followed by your other resistance modules that are penalized normally). If you fit a DC and a Reactive Armor Hardener, the one with the lesser resistance will only be 86.9% effective.

See also Module Stacking and Speed Modules.