Here is an excerpt from Black Library's The 13th Black Crusade. It gives a fairly good number on how much you need to kill a planet. Still, even one Nova Cannon shot should herald unimaginable catastrophe for a planet, but Exterminatus means that you're SURE nothing whatsoever has survived there.
"The world of Saint Josmane's Hope is to be destroyed.
The order was issued in closed session at z2;zo local time: late last night. Present at the meeting were representatives from all major arms and services operating in the Cadian system, including the Lord Marshall, Procurator General, Lord Provost and Comptroller Majoris of the Departmento Munitorum, representatives of the Commissariat, the Imperial Navy, the Adeptus Mechanicus, the Adeprus Ministorum and the Adeptus Terra. Furthermore, Lord Grimnar amended, having returned from his actions against the raitors of the Alpha Legion, as did two agents of the Holy Orders of the Emperor's Inquisition. I was present in my capacity as senior general staff officer. The meeting began with my appreciation of the strategic situation in the Cadian system. Our enemy was assaulting every world in the system with unprecedented fury with Solar Mariatus faring particularly badly.I n my opinion, Saint Josmane'sH ope - which was all but lost to us - was being used as a staging point for Chaos forces. Should it fall entirely from our control it would soon be used in this role, so that the flow of enemy forces and material into the system would become an unstoppable tide. Recapturing the world was, I concluded, a fool's errand. Creed proposed its destruction. Though none in the united council took such a proposition lightly, the approvalt o proceedw as unanimous. And so the business of the murder of a world was debated. Of the options open to us, each in turn was discussed at length, and each in turn was rejected. Exterminatus was beyond our resources in the time availableto us; the agentso fthe Inquisition confirmed this and said that their ships that carried such weapons were many weeks distant. Grimnar too confirmed that none of the available vessels of his fleer carried cyclonic torpedoes, and neither did any Astartes ship within range. The Navy put forward the plan of instigating a massive nova cannon bombardment of the world, in the hope of causing a degree of tectonic instabiliry, but all available intelligence suggested that the enemy fleets blockading Saint fosmane's Hope's orbital space were too smong. The bombardment vessels would not survive to launch a single.shelll,e t alone the hundred or more requiredt o complete the task."