On WWW if you make a thread with post-Unending F-304s people will consistently talk about the gigaton range plasma weapons as if that was an unimpeachable fact. But really it is more muddled than that. All of the actual feats for Energy weapons that I know of don't get into what people would call "nuclear weapon scale" unless you flat out ignore parts of the feats.

There are the few "easily" quantifiable feats for SG energy weapons hitting a "neutral medium" (I.e. stuff that lacks possible exotic properties) that I know of,

• The Ori bombardment which destroyed the Dakara Weapon (Decent showing in the space view but, when viewed from the ground it clearly isn't very powerful).

• The Ori bombardment of the some village (SG-1:Line in the Sand). (Similar to the Dakara feat, we see a good showing in the orbital shot but when we see the effect on the ground it is... pretty pathetic)

• Baal's bombardment of Earth in Stargate Continuum (Which is basically the same as the other two)

  • There are times where it clearly isn't the megaton range weapons that a lot of people seem to claim for the Ha'taks.
  • But there are shots from space where the "explosions" are far larger than what is being shown on the ground.

• Anubis' ship's shots that were missing the Prometheus and hitting Antarctica (Lost City) didn't cause kiloton scale fireball like we might expect from even multi-megaton range weapons.

• The Pegasus Replicator's beam weapon that was burning through Atlantis' shields but didn't instantly punch through an asteroid and only made relatively small steam geyser when it tracked across the planet. (though his one is kind of weird for other reasons).

There are a few that I can't remember if there are quantifiable showings (that /u/doctorgecko pointed out),

• I would have to look at the siege of Atlantis episodes again but if the Wrath hives missed any shots then that would be another example.

• The first episode of SG:U where the Lucian Alliance bombard the planet.

• Yu bombarding Anubis's mothership while it was over Jonas's home city at the beginning of season 7.

And for the record I am not including superweapon feats in with regular energy weapon feats.

Of these you should notice a pattern, when viewed from orbit they are pretty good. When it switches to the close up perspective they could easily be replicated and exceed by any decent modern military.

So really to get any kind of kiloton scale firepower out of these feats you would need to flat out ignore the planet side shots.

The ways to get "clear" gigaton range weapons is based on scaling off of their nukes.

Which generally have pretty fucking fantastic feats. They also have a number of dialogue sources that routinely put them in the high megaton to gigaton range.

You do something like this, take the Ha'tak from Season 1 that tanked two, one gigaton nukes to it's shields with no issue. Then you take the Ori beams punching through two Ha'taks (which have been greatly upgraded since season 1) to say the beam must have exceed ~two gigatons of energy to bring down their shields and punch through the ships like that.

But really Stargate nukes are consistently shown to be far more impressive than their energy weapons. So I do not think this kind of scaling is 100% valid.

It would be like a character whom has no objective feat anywhere near planet busting, yet they are able to consistently hurt people who can tank planet busing attacks.

So overall I really don't think Stargate ships throw around as much firepower as most people on WWW seem to think. But I also don't think we should base their firepower entirely on the planet side showings as they don't make sense in the setting considering how weak they are. Dichotomies like what their energy weapons display certainly make finding a "right" number difficult.