CPU and Power Grid Edit
Merely measure how much stuff you can use. Everything has a CPU and Power Grid requirement. Some have more in one or the other, some are good and bad in both. You'll learn how to balance the two as you get more experience and decide what tools you like to use best. For now, these are the boundaries that you operate within when mounting things on a ship. You might have room to mount a battleship-class canon, but you probably won't have the CPU or Power to pull it off. Don't be afraid to experiment, however. Even if something is not 'meant' to be used on a class of ship, with clever balancing and the right support equipment, it's still possible to bend the lines. Every ship has its own CPU and Power Grid attribute, which can be modified with equipment.
Training levels in the Engineering Skill can increase the available Power Grid in all ships a pilot owns. Reactor Control Unit, Power Diagnostic Unit and Micro Auxillary Power Core modules can also increase available power grid, at the expense of Low Slots.
Training levels in Electronics skill can increase available CPU. CPU fitting can also be improved with the various 'Upgrades' (Weapon Upgrades, Energy Grid Upgrades, etc) skills, which generally lower the CPU requirement for specific modules of that type. Co-Processor modules can also give more CPU, in exchange for Low Slots.
Your ship also has a Capacitor. When you go to warp or use a piece of equipment (which we call 'modules' in this game), your Capacitor gets drained. It recharges on its own, and there are modules that can modify most aspects of the Capacitor. Every ship has its own unique Capacitor attribute, just like CPU and Power Grid. As the CPU and Power Grid govern what you can put on your ship, the Capacitor dictates how often you can use modules and warping.
There are several ways to improve capacitor recharge and capacity, including Cap Recharger, Cap Power Relay, Cap Injector, Nosferatu (or Vampire) and Cap Battery modules, and most of the Engineering section of the skills list help with recharge, consumption and capacity of the ship's capacitor.
Capacitor is life in combat; once the ship's capacitor is empty, most ship functions cease, including weapons, repair and the ability to warp away to safety, so if in doubt over what to put in that extra low or mid slot, go with a Cap Recharger or Cap Power Relay.
Power Slots Edit
In addition to these specifications, every ship has three levels of power slots: high, medium, and low. Every piece of equipment fits into one of these, so how many a ship has is important. If you have the CPU and Power Grid to use something, you might not have enough slots to use alot of them. Every module fits into a high, medium, or lower power slot. Here is a general summary of what kind of equipment goes into each slot.
The arrangement of slots in a ship is fixed and cannot be altered - to improve the number of slots of a given type, a new ship will be needed.
These are usually the biggest Capacitor users. Most high slot modules are weapons of some type. Some are specialized equipment, like Vampires (they drain Capacitor energy from an enemy and give it to you), and Smart Bombs (which explode in a radius around your ship). You can only mount a weapon if you have a free high slot, as well as the right hard point.
Medium modules are usually activated. They can cover a wide variety of non-damage applications. Shield rechargers use capacitor energy to charge your shields faster. Afterburners let you move faster for a short time. There are some modules that help increase your defenses. Another big use of Medium slots is Electronic Warfare modules. These are modules that do a variety of non-damage combat activities. You can jam a ships sensors, or disable its turrets with these modules, for example. You can also mount modules to defend against Electronic Warfare in the medium slots.
Low power slots often do not require activation. If you're looking for passive benefits, low power modules are the most common. You can increase certain defenses, expand your cargo hold, boost your CPU, just to name a few. A ship with many low power slots may not have obvious fancy equipment, but it will probably have a lot of passive benefits.