Power factor is really simple. Bullet weight x Velocity
you'll measure bullet weight by grains, typical weight for 9mm is 115gr, 124gr and 147gr, 45 ACP 200gr and 230grs.
Velocity is measured using a chronograph, shoot 5-10 rounds through a chrono and get the average feet per second (FPS).
As an example, I'll use my 45 ACP load, 230gr, average fps is 750.
230x750 = 172,500 or 172pf.
Depending on which game you're playing, IDPA refers to it as 172,000pf and USPSA 172 pf.
There are two types of power factor that are used in competition shooting : MAJOR and MINOR.
Minor is 125 to 164 pf, Major is anything greater than or equal to 165. so my 172pf load makes Major powerfactor. If you're shooting production, you only have to worry about making Minor pf, so anything above 125pf.
hope that helps.
here's a wiki entry on PowerFactor : http://en.wikipedia.org/wiki/Power_factor_(pistol