each knob on the NXP-201 can handle 18w total. a standard 80mm fan takes about 3-4w. the 80mm tornado takes about 9w, the 92mm tornado takes about 12w. (I should know, I have both of them within arms reach, and I'm holding the 80mm tornado in my hand right now...)
when you do this stuff, look at wattage (or amperage).