The description of the damage wouldn't actually rule out very low kilotons.Jedi Master Spock wrote: Darksaber gives gigajoules for a SSD on full power.
Technically, it is possible to pick most if not all anti-ICS references and observe that they could easily fit with the gigajoule/terajoule range, indeed. But there still are some references that point to higher figures. For example, the "Kachow" - can't remember the comics name - on which I pointed out that it had to be in the megaton range. Not necessarily high, and I suggested that they had a charged shot. We don't know how long it takes to charge, and if it's anything close to a SPHA-T's implied charge time, it could be long.Karen Traviss makes smaller ships sound in the range of nuclear weapons. And "kilojoules."
We have megatons in the EU, and tons in the EU. It's not particularly consistent - the range within the EU is at least 7 orders of magnitude just in the novels (and easily 12 including the ICS). Within the movies themselves, we really have much less inconsistency - 4, maybe 5 orders of magnitude for cruisers at worst between different reasonable intepretations. Realistically, if we use consistent models and stick by them for all incidents, <3 orders of magnitude of variation happens in on-screen evens.
We have that puzzling recoil systems which can withstand an explosion in the giga-tonnage range from "Slave Ship" I think - for which I'd like to hear a warsie explain why a cannon would have to cope with a gigatonne level explosion at all.
There is the near 1 KT yield for a typical X-wing torpedo from the Technical Journal, which implies that capital ships should easily be over that. Mind you, a 1KT fusion warhead would make fireballs 50-60 meters wide, which would work well in SW.
For the kilojoule, it could easily be near megajoule firepower fired on some stutter mode. It would easily bring each bolt near the power of the comparatively weak bolts of the snow speeders.
Notice that there are good demonstrations in the movie. The turret on the back of the airspeeder in TPM, that blasts an AAT in Theed, isn't weak. The transportable gun, barely than a M60, attached to the hand rail of Jabba's barge in ROTJ, dealt lots of damage to the skiff.
We have the pistols of main characters who apparently like to boost their firepower (Solo, Boba Fett).
We could also scale capital ship firepower from Slave-I's midguns, as seen in action in AOTC. Even the recent Clone Wars episode with young Fett trying to kill some Jedi had a firepower in that range.
There also are the higher class thermal detonators from the EU which could blast the basement of Xizor's palace.
Basically, I believe we can consider that Imperial warships are capable of a very few single megaton shots, but that requires long precharge times, which go as far as being one hour long. Therefore, shots of a couple megatons, say 5, would require ~3600 x 1.4 KT. That's for an ISD. I suppose it could greatly overheat the cannon in question.
Figures could be a bit higher as well. With greater terawatts of firepower and lower charge times, you wouldn't look at cannons being so underused that they routinely fire at less than a thousandth of what they're capable of without blowing up.
Figure cover such a range that if it were slimmed down a difference of two orders of magnitude, it would obviously fit in the terajoule range.