Lesser Power

Assumed levels: 85 vs. 85
Resistances
Racial: 85, 60 or 0 (Blood Elf/Gnome, Night Elf/Tauren, Draenei/Forsaken (85), Worgen (65))
Buff: 97 ([spell=20217]/Mark of the Wild)
Aura: 195 ([spell=8184]/[spell=19891]/[spell=27683]/[spell=20043])

Plus stackable [spell=6117]: 45

These are non-stackable (with the exception of Mage Armor), so the most spell penetration you will need would be against a target with mage armor and a resistance aura, a total of 240, or 195 against non-mages, dropping down to 142 (mage)/97 (non-mage) if you're using Arcane spells.

This of course does not take into account procs or on-use effects such as [item=62471]

(Edited - [spell=7302] no longer provides resistance as of patch 4.1)

Great info, thanks! All one really needs then is a pvp back like [item=64732] along with Lesser Power and you are pretty much set resistance wise. I'm constantly running into resistances in Tol Barad and its a huge headache. Your post just saved me a lot of math and legwork, thanks!

Prior to patch 5.0.4 this used to provide 70 Spell Penetration and was named [Enchant Cloak - Greater Spell Piercing]. Due to Blizzard removing spell resistances from the game this was changed to provide PvP Power.

Similarly the TBC (don't get fooled by the tooltip of the enchant, check the actual effect of it) and Wrath Spell Penetration enchants have been changed.