(This will be a short one; I was planning to do a piece on server ticks today, but I need a few more days to work on an interactive demo for that, as its a slightly tricky topic.)

What is a Micro Auxiliary Power Core, and when should you use it?

There are three general categories of modules in Eve that can increase the power grid on your ship for fitting mods. All of them consume a fixed amount of CPU, and add grid:

The Reactor Control Unit (RCU) increases your power grid by a percentage — +10% for tech-1 modules, +15% for tech-2.

increases your power grid by a — +10% for tech-1 modules, +15% for tech-2. The Power Diagnostic System (PDS) increases power grid by a smaller percentage (+5%), but also gives you bonuses to capacitor size, capacitor regeneration rate, and shield size/regen rate.

increases power grid by a smaller percentage (+5%), but also gives you bonuses to capacitor size, capacitor regeneration rate, and shield size/regen rate. The Micro Auxiliary Power Core (MAPC) increases power grid by a fixed amount: between +10 and +13, depending on meta level.

The distinction between percentage and fixed amount is mainly important for frigates and destroyers. An Atron has between 37 and 46 MW of power grid to fit modules (varying with skills); a +10% bonus would only yield 3.7 to 4.6 grid, not enough to make a significant difference. Adding an absolute +10 grid, on the other hand, is a significant improvement. The same is true for most destroyers. But once you start working with cruisers, an RCU (or even a PDS) yields dramatically more grid to work with.

As a general rule of thumb, if you’re looking for more grid, you should only use MAPCs for frigates and destroyers, and only use RCUs/PDSes for cruisers and larger hulls. [1]

In fact, there are actually very few modules in Eve that add absolute bonuses to a ship attribute/stat, rather than percentage boosts:

MAPCs add a fixed amount of power grid.

Capacitor batteries add a fixed amount of capacitor.

Drone Link Augmentors (and equivalent rigs) add a fixed distance of additional drone control range.

Armor plates add a fixed amount of armor HP and mass.

Shield extenders add a fixed amount of shield HP and signature radius.

Signal amplifiers and auto-target-lockers add a fixed bonus to your max number of target locks.

Propulsion modules (both ABs and MWDs) add a fixed penalty to mass.

Data/relic rigs add a fixed bonus to virus coherence.

(Most of these absolute boosts are particularly beneficial when used on “undersized” ships — which is why Stabbers with 100MN MWDs are used for bumping, and why cruisers tend to use large shield extenders and 1600mm plates. [2])

MAPCs were the only “Micro” module whose blueprint continued to exist after Red Moon Rising; throughout most of the game’s history, only the plain Tech-1 module and some meta versions were available. Navy versions were added during the introduction of Faction Warfare in 2008 (as part of the Empyrean Age expansion), and the Tech-2 version was added during Crucible.

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: The one exception to this is when you're flying a shield-tanked frigate with unused low slots. The PDS is the only low-slot module capable of increasing raw pre-resist shield HP; if you have absolutely nothing better to fit in a low slot, you might consider a PDS. In practice, there's almost always a better use of that low-slot; the main frigates that use PDSes are shield-tanked tacklers such as the Hyena and Keres.

: It's also that absolute boosts are generally not subject to stacking penalties. In fact, RCUs and PDSes are one of the very few modules that are not subject to stacking penalties.