"This is the commander of the U.S.S. Enterprise. All cities and installations on Eminiar 7 have been located, identified, and fed into our fire control system. In 1 hour and 45 minutes, the entire inhabited surface of your planet will be destroyed. You have that long to surrender your hostages."
Combined area of largest metropolises on Earth is 134 141 km^2. E-D has 250 photon torpedoes, and due to only taking largest cities as norm, I will ignore phasers. That gives 357 square kilometers per torpedo, or 250 circles with radius of 13 kilometers.
Area of urban areas on Earth is 1.5% of land mass area or 2 234 100 square kilometers - or 8 936 km^2 per torpedo - which is area of circle with radius of 53.3 kilometers.
With Wong's calculator, first value gives us 120 megatons and the second 8.1 gigatons per torpedo - both are low end values, but are balanced out by not taking phasers into account. However, first value takes only few largest metropolises on Earth, and is low-end value. Considering that quote is from 23rd century, and that Enterprise-D probably carries much larger number of torpedoes than Constitution class (or much more powerful torpedoes) due to Galaxy class being 8 times larger in volume, second value might also be conservative - however, it is balanced out by fact that Eminiar 7 might have smaller urban population than modern-day Earth. Following screenshot, however, suggests over 11 000 000 square kilometers of urban area - or 250 circles each with radius of 118 kilometers, or 90 gigatons per torpedo, assuming all damage is done by torpedoes - I doubt more than 50-75% of damage will be done that way.
However, this quote is interesting.
Kirk: "We make the real thing, Councelor. I could destroy this planet."
It could mean that Enterprise can destroy every inhabited area on planet.
CONCLUSION: 23rd century photon torpedoes are between 8 and 50 gigatons in yield.
UPDATE: GO24 firepower is confirmed by episode "Whom gods Destroy". Outpost is covered with planetary shield and it is stated that Big E "can blast throught that shield but only at cost of destroying Captain Kirk, Mr. Spock and every other living thing on Elba 2." This seems to suggest that planet would be totally devastated even by collateral damage of blowing throught planetary shield, which might bump up firepower from GO24 as much as OOM.
Sulu: "We can't beam anyone down sir. Forcefield on planet is in full operation and all forms of transport into asylum dome are blocked."
Scotty: "We could blast our way throught that field but only at risk of destroying captain, mr Spock and every other living thing at Elba 2."
McCoy: "How could we be powerful enough to destroy a planet and still be so helpless."
Also, in DS9 episode "Broken Link" Garak states that "we have firepower on this ship to turn that planet into smoking cinder". He might have been tailor, but he was Obsidian Order operative and Worf does not contradict him.
Dialogue here:
WORF: Garak. Just as I thought.
GARAK: Don't tell me. I overlooked one of the security monitors.
WORF: You were trying to override the launch controls for the quantum torpedoes.
GARAK: I was hoping to gain control of the phasers as well. I just hadn't got around to it yet. Don't you see? We have an opportunity here. A chance to end the Dominion threat once and for all. We have enough firepower on this ship to turn that planet into a smoking cinder. Personally, I think that would be a very good thing.
WORF: And what about Odo, and Captain Sisko and Doctor Bashir?
GARAK: They'll die. And once the Jem'Hadar ships realise what we're doing, so will we. But what are our lives compared to saving the entire Alpha Quadrant?
WORF: We are not here to wage war.
GARAK: I'm not talking about war. What I'm proposing is wiping out every Founder on that planet. Obliterating the Great Link. Come now, Mister Worf, you're a Klingon. Don't tell me you'd object to a little genocide in the name of self-defence?
WORF: I am a warrior, not a murderer.
GARAK: What you are is a great disappointment.
(They fight. Worf finally gets the upper hand.)
WORF: You fight well for a tailor.
Base Delta Zero
"Base Delta Zero is the Imperial code order to destroy all population centres and resources, including industry, natural resources and cities."
I will use that quote to calculate BDZ energy requirements. NOTE: BASE DELTA ZERO IS LOW CANON AND AS SUCH IS OVERRIDDEN BY G CANON AND CALCULATIONS DERIVED FROM G CANON, ESPECIALLY CONSIDERING AMOUNT OF SPECULATIONS.
Timeframe: Base Delta Zero has been said to be "matter of hours". This means it takes more than one hour, but less than full day. Probably less than 6 hours, since six hours is "half day work".
We don't know how many ships are involved in this, but we can take some guesses:
"System bombards are used when the Empire would rather completely destroy a world rather than see it fall into Rebel hands."
"System bombard contains an average of 100 ships divided between three bombard squadrons and a light squadron. If an admiral feels that force superiority has done less than a thorough job of removing hostile craft from the system, a system bombard squadron will be augmented with ships from the light squadron."
As per the Sourcebook, the 100 ship fleet may include a few ISDs.
Task Force 58 at one point had 9 aircraft carriers, 7 fast battleships, 6 heavy cruisers, 4 light cruisers and 28 destroyers. Given that ISD's are Imperial battleships/carriers, I will take that "few" means anywhere from 12 to 29 Star Destroyers.
As a wild guess, I will assume 5 000 000 square kilometers destroyed. I still need to obtain ROF of Imperial turbolasers.
From Tantive IV pursuit scene, light turbolasers seem to have ROF of 3 shots per second. Medium ones seem to be able to fire 1 shot every 3 seconds. There seem to be 8-10 medium TL emplacements, so we have 2-3 shots per second. Heavy TL might be able to fire 1 shot every 10 seconds.
EU gives 72 heavy turbolasers and 19 medium turbolasers - so we have 7.2 heavy and 6.3 medium bolts per second.
I will give 3 hours as timeframe for BDZ (wildly generous, but there we go). In that timeframe we can count 25 920 heavy and 22 600 medium bolts. I will take heavy turbolasers as being 100 times stronger than medium ones - so we have 26 146 equivalent heavy TL bolts. Each would have to destroy area of 191 square kilometers, or equivalent of circle with radius of 7.8 kilometers.
I will use Wong's calculator, which gives us 24 megatons for air-blast radius with total fatalities, and 1.4 gigatons for minimum fireball radius.
Medium TL bolts therefore should be 240 kilotons to 14 megatons per shot.
CONCLUSION: Heavy turbolasers as based on BDZ are 24 megatons to 1.4 gigatons, and medium ones are 240 kilotons to 14 megatons.
