Got some info from DITL.
Apparently, a regular photon torpedo has a firepower of 18.5 laughable isotons (whilst they may be the same as tons in deadweight tonnage, they're 2.48 megatons in explosive yield), which amounts to 45.88 megatons.
However, a quantum torpedo is at around 71.8 isotons, making it at a less laughable 178 megatons. A type VI photon is stated in 'scorpion' to be 200 isotons, making it a relatively whopping 496 megatons.
B'Elanna's little war machine, dreadnought, is calculated at 16,879 isotons, putting it at 41,860 megatons. Finally, a Borg multikinetic neutronic mine is put at 5,000,000 isotons, leaving it at a huge (on anyone's scale) 12,400,000 megatons.
So there you have it. Standard photon - less than an H - Bomb. Type VI photon - significantly more.
Neutronic mine - BANG!!!!
