The conversion rate for powers to stunts is roughly 2:1, that means a power that's worth 1 refresh is roughly worth 2 stunts. Roughly. I doesn't always work out like that, but it's a good rule of thumb.
Remember though, that you get +2 refresh if you don't take any powers, so you might be able to get more, depending on the powerlevel you are playing at. To keep mortals compatible with supernaturals, we had a discussion about a progressive bonus for the pure mortal template, but I don't know where it is.
But that's a problem we can solve.
*summon greater linkmaster*