"Increases healing done by spells by up to XX."

[Arnulf]

This is something I do not understand. Everyone probably knows this phrase in one way or another: "Increases healing done by spells and effects by up to 31." (Source.)

My naive interpretation of this would be: for every casting of a healing spell I get roughly 31 more health points out of it. But that can't be it, right? I hear that casting time plays a role. Please, can anyone explain this to me, or point me to a place where this is explained? Thank you.

The reason why I'm asking this is that I'm thinking about getting an enchantment on a weapon that has this property. Let's say I've got the Aurastone Hammer (+25 healing) and want to enchant it with Healing Power (+55 healing). I assume the result would be: "Increases the healing done by spells and effects by up to 80 (= 25 + 55)."

But does it really work that way? Maybe I'm wasting a pretty good enchantment because I do not understand the underlying mechanics.

-Arnulf

[hiryuu1115]

I believe the current understanding is that the benefit is fully applied to spells with a base casting time of 3 seconds. As casting time decreases, the benefit decreases correspondingly down to half the benefit for a 1.5 second casting spell. Channelled spells and Instant cast spells are 1.5 second casting spells for all intents and purposes.

[KiloVictor]

I believe the current understanding is that the benefit is fully applied to spells with a base casting time of 3 seconds. As casting time decreases, the benefit decreases correspondingly down to half the benefit for a 1.5 second casting spell. Channelled spells and Instant cast spells are 1.5 second casting spells for all intents and purposes.
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This is the conventional wisdom (some say 3 seconds, some 3.5), but in fact there is a scalar factor which varies per spell. Also, all DoT/HoT spells receive full benefit of any +dmg/+healing items, but it is added incrementally to each tick of the spell. But to know the % of +dmg/healing that will be added to a given spell, you pretty much have to test it out and record the results.

I know on the official Warlock forums there is a post which has empirical values of the % scalar for each Warlock spell, and there are probably similar lists on the other fora.

~Kv