I really, really like the way magic is done in the Night Watch/Day Watch world the more I read it. There's obviously some spellcasters that are better than others (Class level or skill level), and there are some that will never reach a high level (perhaps need a feat to advance their capability). They draw "energy" their power from within themselves and from others (the Light can take it from those that are experience moments of joy, happiness, etc. Conversely, the Dark draw their energy from those that are in despair, anger, etc.) — power points.
There are restrictions that this drawing of energy can occur (especially on the side of the Light). There was a scene in which Anton (the hero from the movie) gets permission to draw energy and uses it to basically keep himself from going into the Dark. (Not really, but I don't want to spoil what he really does since it may happen in the next movie). Another scene shows Alisa (or Alice in the movie) expending energy in a fight against the Night Watch, and has to draw some of her own internal energy. Another witch dies because she did the same thing, and Alisa suffers a major setback in her own ability as well.