Game:

Path of Exile 2 (2024)

Duration: 0:00

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The base mana cost of the skill is 1% of your mana per second. In my case this is 17 mana out of 1700 mana pool. Should add 30% of the mana spent to the tempest´s mana drain per second. What it actually does it adds 30% of the base mana cost to the tempest which then starts to MULTIPLY BY ITSELF roughly after 2 seconds. So cast tempest then cast spark once (134 mana cost) and the tempest will drain u roughly 40 mana per second after casting. For some reason after 5 seconds the mana drain goes up EXPONENTIALLY. There is nothing of the sort stated in the tooltip of the skill. It should only add the mana cost of the spell to the base cost of the mana tempest.

The worst you can do is cast CHANNELLING SKILLS. My bonestorm has a mana cost of roughly 250 mana per second. Okay. It casts roughly 3 times per second. In this case the tempest adds correctly around 80 mana to the base cost of the tempest after the first cast. But the second you start channelling bone storm again something happens. The mana cost is around 500-600 mana per second after the third cast. One bonestorm takes about 1,5 seconds to cast (20 projectiles). So it seems the base mana cost of the skill multiplies by itself in the background and then the cast spells are added in the actual base mana cost of tempest which is then again multiplied by itself.

RESOLVED FOR NOW:
Use lifetap! It doesn´t multiply the cost by itself. Lifetap + Inspiration and you are good to go.