"Children on Tatooine tell each other of the dragons that live inside the suns; smaller cousins of the sun-dragons are supposed to live inside the fusion furnaces that power everything from starships to Podracers."
"Space filled temporarily with trillions of microscopic metal fragments, propelled past the retreating ships by the liberated energy of a small artificial sun."
So we know that Star Wars ships are fusion-based (normal, star-type fusion), which limits their maximum output - while maximum theoretical efficiency of M/AM reaction is 40% (rest being neutrinos), fusion has much lower efficiency.
According to this page, energy density of fusion is 576,000,000 MJ/kg, while energy density of M/AM reaction is 89 876 000 000 MJ/kg. With that, fusion reactor using same amount of fuel as GCS warp core is going to produce 156 times less energy, or 20.5 to 81.73 million GW, as opposed to GCS's 3.2 to 12.75 billion GW.
ENDURANCE
Above we have picture of ISD. There is "bulge" clearly visible. While it might house reactor itself, it might also carry fuel.
If we assume it is carrying fuel, we have this situation:
Red circle is 137 pixels wide, as opposed to 990 px for ISD length. That gives us 221 meters for diameter. Using rounded 220 meters figure, we get volume of 5 575 280 cubic meters, for 905 425 433 kg of fuel (assuming ordinary liquid deuterium). Assuming 89 kg of fuel soaked per second, we get 117.75 days before refueling. Assuming 1 year of operation before hitting gas station, we get 26.35 million GW standard power output, high-end.
On the other hand, if ISD is using ultra-dense deuterium (althought from canon it seems that it is not using either deuterium or its dense brother, but something more like diesel or alcohol, which would lower its energy output to ) we get 4300 years of endurance. However, if we take 1 year as endurance, we need 1297 cubic meters of fuel storage capacity.
However, fuel properties, as seen in movies, throw out either deuterium or its heavyweight cousin right out throught window.
Obi-Wan and Anakin are obviously not freezing corpses, so neither version of deuterium applies. However, any fuel that has hydrogen as part of mollecular structure is in basket, especially if it has oxygen too.
Let's see what we know for now:
a) fuel is not super-dense
b) it is liquid at approximately room temperature
c) it is flammable
d) has hydrogen and possibly oxygen as part of mollecular structure
e) it releases highly flammable but non-toxic gasses
f) it is visually similar to water
First point eliminates super-dense deuterium. Second argument eliminates all gaseous fuels, like hydrogen, deuterium etc. Third option, naturally, eliminates water. Last three points leave alcohol, propane, butane and diesel as most likely candidates.
(and yes, it is fuel):
12 INT. GENERATOR ROOM -- TRADE FEDERATION CRUISER
The TWO JEDI cut their way down several floors into a large generator room. Huge EXPLOSIONS outside the ship have caused several large pipes overhead to break, and fluid is spewing everywhere. The Jedi get up and turn off their lightsabers. ANAKIN dips his hand into the fluid and sniffs it.
Obi-Wan: . . . fuel. The slightest charge from our sabers will send this ship into oblivion. That's why they've stopped shooting.
Anakin: Well, then, we're safe for the time being.
Obi-Wan: Your idea of safe is not the same as mine.They run, EXPLOSIONS rattle the ship, and pipes continue to burst around them, spilling more fuel into the hallway. At the far end, SIX SUPER BATTLE DROIDS drop into the fuel. The SOUNDS OF SHIELD DOORS CLOSING AND LOCKING ECHO throughout the hallway. They pass several large power generators, which are topped with SPARKING excess power dischargers.
Anakin: They're sealing this section off.
Obi-Wan: Six droids coming our way!The last of the DOORS CAN BE HEARD CLOSING in the distance.
Anakin: Keep moving. There must be vents . . . This way.
THEY move along a wall. ANAKIN climbs up the side to a small vent. The fuel gets closer to the SPARKING dischargers.
Obi-Wan: We'll never get through that. It's too small!
They move toward a second vent. OBI-WAN is swimming in the fuel as it reaches to within a couple yards of the ceiling. ANAKIN feels along the ceiling and finds another smaller vent. He closes his eyes and tries to sense an opening, then he moves on. OBI-WAN is forced into hand-to-hand combat with one of the SUPER BATTLE DROIDS. It pulls the Jedi under the fuel. Just before he is about to drown, OBI-WAN disables the SUPER BATTLE DROID by pushing him into an exhaust pipe.
The fuel is up to the Jedi's chins. ANAKIN finds a very, very small metal grate, then pounds on it until the tiny grate breaks loose.Anakin: I found our escape vent.
Obi-Wan: Anakin, this is no time for jokes. We're in serious trouble here.
Anakin: Only in your mind, My Master. Look, no structure. . . .ANAKIN grabs the side of the tiny hole and gives it a big yank, ripping a large panel loose revealing a "man-sized" work shaft. They scramble through it as the DROIDS swim closer.
13 INT. VENT SHAFT -- TRADE FEDERATION CRUISER
The TWO JEDI pull themselves through the narrow vent shaft until they reach a small hatch in the side of the tube.
Anakin: Here's a way out.
As the SUPER BATTLE DROIDS reach the opening in the ceiling and the fuel gets to within a few feet of the power generator sparks, the JEDI work the keyboard on the pressure lock, opening the hatch.
14 INT. SMALL PASSAGEWAY -- TRADE FEDERATION CRUISER
The TWO JEDI climb into a small passageway and slam the hatch shut. They make their way through the ever-shrinking shaft until they reach the end.
15 INT HALLWAY -- TRADE FEDERATION CRUISER
A hatch opens in one of the main hallways of the Trade Federation Cruiser, and the JEDI squeeze out, SLAMMING the hatch. Behind them, ANAKIN seals the hatch with his laser sword.
Obi-Wan: That won't hold when the fuel hits those power dischargers.
Anakin: The blast will break the hull. This side's pressurized.
Obi-Wan: You still have much to learn, Anakin.
16 INT. VENT SHAFT -- TRADE FEDERATION CRUISER
The SUPER BATTLE DROIDS climb up the vent shaft. SUPER BATTLE DROID R77 and SEVERAL OTHER DROIDS wait in the generator room as the fuel continues to rise toward the power discharger.
Super Battle Droid R77: I have a bad feeling about this.
17 INT. GENERATOR ROOM -- TRADE FEDERATION CRUISER
The fuel hits the SPARKING power discharger, and there is a HUGE EXPLOSION.
18 EXT. TRADE FEDERATION CRUISER -- BATTLE
A GREAT EXPLOSION and a flaming gas cloud spray out of the side of the Federation Cruiser.
19 INT. HALLWAY -- TRADE FEDERATION CRUISER
A large bulge appears in the wall around the sealed hatch as the EXPLOSION hits. OBI-WAN jumps back, then stands amazed.
Obi-Wan: All right, you win. I have much to learn. Let's go!
ANAKIN grins at OBI-WAN, and they run down the hallway.
On the other hand, it is entirely possible that this fuel is used simply to power ion engines in starfighters (it is near hangar, and ion engines must have something to ionize).
POWER OUTPUT
That, however, returns us to beginning. It seems that only way to calculate power output of ISD's reactors is to calculate output of its engines.
Picture above is screencap from AotC, showing Acclamators taking off. Acclamators are around 1 kilometer away, as seen below.
This link gives us 1st degree burns at 4.3 kilometers for 20 kiloton bomb. Due to bomb having burst output, while output of engines is continuous, we have 100 tons of TNT per second as standard continuous output of Acclamator's engines. Total reactor output might be 1 to 10 kilotons per second, or 4.2 to 42 terawatts. Total output might be around 60 TW. Venator might have 80 to 120 TW output, with ISD having 180 to 600 TW output. Combat output is possibly higher, althought it is unclear how much higher - 600 TW is 143 kilotons per second, and ISD has total output of weapons as 197 kilotons per second, so combat output might be around 300 kt per second, or 1255.2 TW.
"'You ready for some power?' Luke asked Artoo, who was patiently waiting for his own form of nourishment. Luke took a small fusion furnace from an equipment box and ignited it, welcoming even the tiny glow thrown off by the small heating device, then took a power cable and attached it to Artoo [...] As power radiated through Artoo's electronic innards, the stout robot whistled his appreciation." - TESB novelization
We are later told that unit is fission-based:
"Luke turned around to see the little droid standing forlornly next to the miniature fission furnace." (Ch. 8)
However, it confirms nuclear technology as being something standard in Star Was universe.
