In the core rulebook, page 120, creating a guard circle, it is stated that the creature's Power level must be greater than the spell level to overcome the protection.
When we speak of level, do we mean the final level of the spell or the base effect?
The spell's level must be equal or higher than the Might of the creature to be warded against, but do note that the spell also needs to Penetrate the MR, so essentially, your Casting Total should be Twice their Might.
While it is not clearly stated anywhere, General Wards all use the 'Ward Spells Guideline' as pointed out by Raccoon which means their actual perpetrator is effectively (level + 3 Mags). The way described for calculating it however is to modify the level by the change to R/D/T from its default T/R/C
A Personal/Diameter/Individual Ward with a level 10 would affect beings with 20 Might or less and its Formula would be "Base 20, -1 Personal, -1 Diameter".