It won't drain if it doesn't hit anything, I think there is a set cost, but on top of that it drains as well. Just a sec *runs to hero builder for info* it's 20.8 end cost, then when it hits all your end is drained. The amount of damage is set, just make sure you have at least 21 end when you fire it off.
It still drains all even if you don't hit anything.
no it doesnt i fire of inferno randomly in atlas when i feel like it, unless something has changed whilst i was in sapin
No, it doesn't. Whether this is a bug or not, I've seen countless fire blasters randomly inferno-ing and keeping all their end (aside from the 21 cost)
It costs some endurance to actually fire it off, meaning that you have a chance to weaken a tough mob with Fire Ball, Rain, and Breath, before diving in anc clicking it. As long as it hits, it will do the same damage as it would've done with full endurance, and you will still be drained.
Thanks for all your replies, most helpful 
Actually all of the nukes work like that, nova and blackstart work the same I know that.
Yeah, I kinda assumed that would be the case, although Fire is the only 'nuke' capable blaster I have... I do have some AR/Devs, but of course they don't get a nuke in the commonly accepted sense.
I can confirm that Thunderous Blast doesn't drain all your end if you miss as well.
Inferno - when you use it, am I right in thinking that it drains you of all of your endurance, regardless of exactly how much you had when you activated it?
I assume it still has an actual fixed endurance cost to activate just like most other powers, i.e. if you're low on End it won't fire if you do not have the minimum End required?
Assuming you do have just the bare minimum endurance cost to activate it, is the damage it does fixed, i.e. is it affected by how much endurance you had when you activated it, or is it reduced if your Endurance is low?
Thanks in advance for any answers.