The stacking penalty applies for each stat that is being modified. For example, a thermal hardener would not be affected if you also fit a kinetic hardener, but it *would* be affected if you fit thermal plating, or omni-resist plating.

[The precise formula][1] is somewhat complicated, but it can be summarized as:

1. The first module provides 100% of its bonus.
2. The second provides ~87% of its bonus.
3. The third provides ~57% of its bonus.
4. The fourth provides ~28% of its bonus.
5. The fifth ensures your fit gets posted in a "fail fits" thread on the forums.

The order you fit the modules doesn't matter; the one with the largest bonus will always be considered "first".

[Some modules][2] (notably [Damage Control Units][3]) are exempt from stacking penalties.


  [1]: http://wiki.eve-id.net/Stacking
  [2]: http://www.eve-wiki.net/index.php?title=Stacking_Penalty#Exceptions
  [3]: http://wiki.eveonline.com/en/wiki/Damage_Control_II