D&D 5E Overchannel - how does it work?

[Zander]

Starting at 14th level, an evoker (wizard) can overchannel a 5th level or lower spell (minimum 1st level) to do maximum damage.

How does overchannel work in terms of game mechanics?

Do you have to declare that you're overchannelling before you know the outcome of the spell? For example, if you're casting a spell that requires a hit, do you know whether you have hit (or even critted) before you commit yourself to overchannelling? If you don't know, what happens to your ability to overchannel if you miss? Do you lose it for the day, i.e. automatically progress to the self-damaging use of this ability?

Also, what's the best PHB 'to hit' spell to use with overchannel? I'm thinking an upcasted Witch Bolt, though I would understand if the DM ruled that overchanneling only applied to the initial attack, not subsequent rounds of damage.

I searched to see if these questions had been answered before but didn't find anything. Apologies if my search-fu is not what it should be.

 

[Tormyr]

If I were running it, I would have the Overchannel option declared instead of rolling dice. So this would occur after a hit (scorching ray) but before saving throws (for fireball).

 

[jaelis]

I agree, I would declare it at the time damage is rolled.

Interesting question though whether it would apply only to a single damage roll or to all rolls for the spell. From the wording, I'd say all rolls. It looks like the Sage agrees:
http://www.sageadvice.eu/2016/12/17/if-i-overchannel-bigbys-hand/

Best spell I guess would be whatever does the most damage. I think I like an upcast wall of fire, good combo of hitting multiple targets and lasting a while, especially if you can immobilize opponents. I'd probably go for flaming sphere over witchbolt; less raw damage but more flexibility.

 

[CapnZapp]

I'd say the decision point is when you are about to pick up the damage dice.

This means after making an attack roll, since you only reach for damage dice if you hit. As to before or after monster saving throws, I'd leave that up to the DM.

As to whether the Sage agrees with you, Jaelis, my take away is how the Sage makes it clear that the RAI is for you to spend one usage each time you pick up the damage dice, as opposed to only one usage per casting.