Explain Spell Penetration please

[Pelleas]

Okay so sorry for the newb question: Trying to play my mage in PVP as my first caster char. And I read "you need Spell pen so that a missed sheep won't cost you the fight" - But, missing targets isn't that what +hit is supposed to fix? Is spell pen working as arpen? i.e a way to increase your damage output on targets by reducing their resistance to magic?
Also, If I'm only a BG player, how much should I really care about spellpen vs ignoring it and add more sp?

Many thanks guys!

[Deleted]

One point of spell penetration reduces the target's magical resistances by one point. Nothing less, nothing more. Any spell pen you have over your target's magical resistance is wasted, just as hit is wasted if you are over the hitcap.

As such, you need 75 spell penetration to get around Mark of the Wild, and 130 spell penetration to ignore the effects of paladin auras or other resistance buffs.

[Tassir]

To put this in a mage perspective.

MOTW is the ONLY source of buffable arcane resist. Pally auras/totems DO NOT stack with MOTW, and there is NO arcane resist aura.

arcane spells need 75 spellpen to not get resisted. frost/fire spells need 130.

Mages are the sole exception, as we innately get 80 resist with magic absorption, 40 resist with mage armor and both stack with MOTW.

Its been said before, but i'll reiterate: mage vs mage fights are VERY RNG.

[Swizzle]

you forgot warlocks and their fel hunters.

also, when people say "missed sheep", they mean a resisted polymorph. the higher a players resistance to a given school of magic, the less damage they take from that school and they have a higher frequency of fully RESISTING spells of that school. fully resisted spells are spells that go off but fail to have their effects applied since they were essentially negated.

[Goliethlol]

If you're familiar with physical stats and gemming, consider spell penetration to be the expertise equivalent.

Certain classes (Paladins, Mages, Locks, Druids for mages) have resistances due to abilities or talents, and by having those resistances it gives X% chance to fully negate a spell. Spell penetration removes their resistances on a 1 - 1 basis. IE: if the target has 100 Resistances and you have 60 penetration, they essentially have 40 Resistance when it comes to calculating the resist chance.

The general rule of thumb as a mage is to get 130 Spell Pen.