Spell Power

Spell power rating determines how much damage you deal with spells. In most cases, each point of spell power increase increases the base damage of your spell by 1%. Various effects can contribute to your spell power, all effects are added together.

Spell power was introduced in Update 14 and replaced previous spell damage amplification that involved addition and multiplication.

Spell power applies only to spells that cause hit point damage and spells that heal hit points.

Spell power affects Spells and SLAs, but does not affect magical damage effects on weapons from augments, filigrees, crafted enchantments etc. unless the description specifically indicates that it does
 * e.g. +1d6 Light Damage per hit from a crafted Coruscating effect is +1d6 at 0 Light spell power and still +1d6 at 100 Light spell power

Spellpower only affects your own spells or SLAs, it does not affect spells that you cast from scrolls, wands, clickies, etc.

Universal Spell Power

 * Fully stacking

Class and racial enhancements

 * Everything fully stacks

Metamagic feats
(spell power increases from metamagic feats stack)
 * Empower Healing Spell +75
 * Empower Spell +75
 * Maximize Spell +150
 * Intensify Spell +75

Standard magic items

 * Equipment bonus from items with enchantments like Potency or Combustion. The item enchantment name corresponds to the type of spells the spell power applies to. The equipment bonus on randomly generated loot increases on higher level equipment. (See Thaumaturgy for an overview table.)
 * Insight bonus from or ; since Update 29 often also on random loot
 * Quality bonus on named items
 * Implement bonus from caster weapons (equal to weapon ML for newer weapons; equal to 3 * enhancement bonus of the weapon for older weapons)
 * For older weapons, effects that increase the enhancement bonus also increase the implement bonus, e.g. Artificer Enchant Weapons or Druid Magic Fang.

Rare / unique magic effects

 * Alchemical bonus from clickies and potions, e.g. Inferno. The item enchantment name corresponds to the type of spells the spell power applies to.
 * Artifact bonus from Draconic Mind set bonus
 * Psionic bonus from, , or Planar Conflux set bonus
 * Mythic bonus: Mythic Boost items
 * Reaper bonus: Reaper Power Boost +3

Epic Destiny bonuses

 * Bonuses from Epic Destinies (e.g. Dragon Heritage) usually stack with everything else.

Skill bonuses

 * Heal increases your Positive and Negative spell power
 * Perform increases your Sonic spell power
 * Repair increases your Repair and Rust spell power
 * Spellcraft affects everything else

Boosts

 * Action Boost: Spell Power Boost
 * Action Boost: Skills
 * Wellspring of Power
 * Reaper's Power memento: +45 Profane bonus
 * Tainted Spellcasting: +25 Spellpower
 * Reborn in Light: +100 Light Spellpower
 * Draconic Fury: +50 Spellpower

Exceptions
Some spells receive limited (usually half) benefit from spell power:
 * Heal: 50%
 * Mass Heal: 50%
 * Necrotic Touch (Pale Master): 50%
 * Necrotic Bolt (Pale Master): 50%
 * Necrotic Blast (Pale Master): 50%
 * Summon Archon (Favored Soul): 50% unnerfed in Update 39
 * All Runearm Shots: Between 50% and 80% unnerfed in Update 36

Heal and Mass Heal spells still receive full benefit from the Empower Healing Spell feat.