Power Generation and Distribution
Several power generation systems exist on most types of starships. In general, there are three main power generation systems: (1) the Matter/Antimatter Reactor Assembly (M/ARA); (2) The Impulse Propulsion System (IPS); and (3) the Auxiliary Power System (APS). When a starship possesses all three types of power systems, the primary source of power is the M/ARA, the secondary power source is the IPS, and the APs acts largely as an emergency backup and supplement to the other two systems. Power from all of these systems is distributed throughout the ship via the Electro-Plasma System (EPS).
Matter/Antmatter Reactor Assembly: The M/ARA is the primary source of shipboard power–both for the operation of ship systems and for the operation of the Warp Propulsion System (WPS). M/ARA maximum output is determined during the design stages by the power requirements for the WPS as these requirements outweigh other power considerations by several orders of magnitude. Standard ship hotel and tactical loads represent only a small fraction ( less than 1e6 MW) of the total output of the M/ARA (which can exceed 1e13 MW for larger classes of ships such as the Galaxy class).
As the name implies, the M/ARA generates power from the mutual annihilation of matter and antimatter under controlled conditions. Matter and antimatter from fuel storage is fed into the M/ARA at varying ratios (this ratio of matter to antimatter is generally referred to as the intermix ratio) and the reaction cross-section radius varies between 2.1 to 9.3 cm (from low power to high power–a larger cross-section radius means more annihilations per unit time). The system operates in one of two modes: Mode 1is used for lower energy levels delivered to the EPS for sublight operations. The dilithium crystal is used to moderate this reaction and the Dilithium Crystal Articulation Frame (DCAF) aligns the dilithium crystal so that two of the edges of one of the crystal facets are parallel to the reaction cross-section radius. The intermix ratio for power generation is 10:1 (10 units of matter per unit of antimatter). Mode 2 is used during warp propulsion in order to create the Critical Warp Pulse Frequency (CWPF) for warp propulsion operations. DCAF orientation is controlled in three axes so as to place the mathematical collision point 20 angstroms above the upper facet of the dilithium crystal. The actual reaction takes place at the chamber centerpoint in either mode.
Note: There is no indication as to the location of the reaction cross-section during Mode 1(whether it is inside or outside of the dilithium crystal, itself). It is most probable that the actual reaction takes place at some distance above the crystal (the antimatter having passed through the crystal on its way to the reaction cross-section).
During Mode 2 operations, intermix ratio varies with the desired warp factor. At idle, the intermix ratio is 25:1. For power generation and warp 1 entry, intermix ratio is 10:1 and the ratio decreases steadily through the warp curve until it reaches 1:1 at warp 8. Warp factors greater than 8 use more reactants, but the intermix ratio remains at 1:1.
At lower power levels and high intermix ratios (used for EPS power generation and distribution) plasma flows from the M/ARA through the PTCs (Power Transfer Conduits) to the EPS, where power is drawn from the plasma via MHD (magnethydrodynamic) generators for distribution throughout the ship as AC or DC electrical power using a standardized electrical distibution system. A portion of the spent plasma is vented overboard in order to maintain system operating pressure within specifications. During high power operations, ship's power demands are met by plasma from the IRCs that is sent to the EPS. The PTCs are isolated from the EPS and the plasma in the PTCs acts as a conductor for the energy released in the M/A reaction in the M/ARA–because the intermix ratio is set at 1:1, little or no plasma is generated during the process (a small amount is generated due to the M/A reaction not being 100% efficient). The energy pulses travel through the plasma, down the PTCs to the warp coils.
Power for particularly high energy ship systems, such as phasers and shields, is provided via direct plasma taps from the EPS for the components in question. Local generators and/or power transformers convert the energy in the plasma into a form that can be used by the components. The level of power required to operate these systems, while low in comparison to warp power requirements, is sufficient to render standard power distribution systems too inefficient and massive for use in these particular applications.
Note: It isn't really possible to say too much that is concrete in regard to power distribution or power requirements for a starship, as little solid information is available in the technical manuals or the episodes/movies. The above represents my best guess–supported by some comments that Rick Sternbach has made in the newsgroup startrek.expertforum.ricksternbach. The largest power loads aboard a typical starship will be: (1) the warp propulsion system (this requires the most power of any system aboard, by orders of magnitude); (2) the tactical deflector system; (3) phasers; (4) structural integrity and inertial damping (SIF and IDF); (5) long range sensors; (6) transporters and replicators; and (7) ship hotel loads (gravity, temperature and air). During STL (slower-than-light) operations, a ship such as a Galaxy class has more power potentially available than it could possibly use or distribute–even with all shields drawing full power and all weapons firing continuously. There is no technical reason, other than a failure of or damage to the distribution or generation systems, why a starship should ever be short of power when not under warp drive. Nor should it be necessary to spout such orders as more power to the shields–since the shields are an autonomous system that will draw the necessary power, up to their peak power load (473 GW per generator), in order to protect the ship.
IPS Power Generation: During warp flight, the IPS is the primary source of power for ship's systems. IPS power is sufficient to provide for all ship's loads (including tactical deflectors and weapons) with a significant excess capacity. During power generation operations, the fusion plasma from the IRCs (Impulse Reaction Chambers) is shunted to the EPS by the IPS Accelerator/Generator rather than being exhausted overboard as per the normal propulsive path. The EPS uses the plasma from the IRCs to generate AC and DC electrical power for distribution to ship's systems (although, as with power generation using the M/ARA, certain components are directly tied to the plasma distribution system due to high power requirements).
Power generation for ship's requirements using the IPS is preferred to using the M/ARA for power generation for logistical reasons (even when the ship is not traveling at warp). Power generation using the M/ARA consumes the ship's onboard stock of antimatter–a resource that is not easily renewable. Power generation using the IPS draws only from the ship's deuterium stocks and these stocks may be replenished without recourse to Starfleet logistical support by collection of free hydrogen via the Bussard Collectors.
IPS power generation is not possible during impulse operations (except for that power drawn from the plasma via MHD in the IPS accelerator/generator) due to propulsion having priority over power generation from the IPS. During impulse operations, power for ship's requirements is normally drawn from the M/ARA.
Auxilliary Power System: Most UFP starships have an auxilliary power system for use in emergencies or when additional power might be required beyond that available from the M/ARA or the IPS. The APS usually consists of one or more IRCs that are not tied into the IPS and that are located at various decentralized positions throughout the ship (in order to minimize loss of capacity due to combat or other damage and to provide power to sections of the ship which have been isolated from the main EPS system due to damage or casualty). APS capacity is usually limited in both total power available and in duration.
Environment
Operations outside the biosphere of a planet requires the maintenance of an environment that is conducive to the efficient operation of the lifeforms that man the ship. Air, gravity, food, waste removal and processing, and protection from radiation and other hazards must be provided to protect the crew of the ship and to allow them to function. A number of systems exist to provide the proper environment aboard UFP starships.
Ship Structure
Cross sectional view of standard UFP starship hull construction here
Transporters
Matter Replication
Tractors