It also just has to do purely with how much power you're tossing about. It's easier to control a leak from a garden hose on low pressure than from a pressurized fire hose.
Think of the Wizard Interference with technology, as a magnetic field with variances in both frequency and power. A wizard is like an EMP, so to speak. just as with an emp, which can be shielded against, one can shield against magic users. Granted magic using humans are ground out when under running water or when they cross a threshhold univited.
My concept is akin to how you shield against emp, the ambient magic energy is going to find the path of least resistance and get ground out. that is also where the magical capacitors come into play. My character beleieves magic works like electricity, and so he quantifies it in terms of it's load potential, path of current, and how it can overcome resistance.
Natural reserve, drawn power, and stored power come into play when it comes to my character casting spells. His naturally reserve is not nearly as high as Mr. Dresden, however I'd imagine he is powerful enough to be a council wizard. What he does with his own power is all done with efficiency in mind. Next is the drawn power, this is taken from nature, spirits, bargains, or in my characters case magic occuring around him or directed at him. Where as a lot wizards use shields against each other, my character has shield spells, but rather harness all the energy being direct at him. Think in terms of akido, an attack is a gift. Physics wise, that is energy to be manipulated, redirected, or stored for later use. finally stored Power, in the books, rituals, charms, areas, and potions all store power. They all have a liited ammount of time to be active for the most part, however that is where i got the idea for magical capacitors. Magic slowly leaks out, so if you build charms in the way that they are designed to slowly drain energy and absorb energy, there is an ebb and flow. You still have energy to work with, but you don't need to worry so much about constantly reinforcing certain effects upon them.
Sorry to get intoo all the electronic their, but it's how I imagine magic works in Butcher's series, outside of the belief thing. Magic is much more complex for mortals than it is for fey and outsiders.