They always start with power/favour equal to their WP, that is the so called equilibirum state. From page 111 in The Player's guide (not sure if it is stated this clearly in the Core set rule book):
"A spellcaster who has as much power as his Willpower score is said to be at equilibrium. It is the default, natural “rest” state for all wizards. All spellcasters trend towards equilibrium, and they are assumed to be at equilibrium anytime their current power is not defined, such as during story mode (in most cases), or at the begin- ning of an encounter.
During the end of turn phase, a character whose current power is less than his equilibrium gains one power. A character whose current power is greater than his equilibrium loses one power, unless he spends a manoeuvre to control the excess, as noted under Excess Power above. Characters also trend towards equilibrium in this way during a Rally Step, gaining or losing power as if it were an end of turn phase.
Note that priestly favour behaves in exactly the same fashion as a wizard’s power, in regards to equilibrium."