HI
here there a analog circuit,is a EDN Magazine article, i think that is very simple for measurement a power factor:
http://www.edn.com/archives/1994/042894/09di3.htm Other circuit:
http://electronicdesign.com/Articles/ArticleID/2642/2642.htmlSee the picture for more details,the first circuit use a ICL7107 chip is a ADC multipurpose with 3-1/2 digit seven segment output
the second circuit, uses optocouplers for the measurement of v-i phase displacement.
With some modifications will can adapted to MCU o Microprocessor for complete measurement and control of power factor.
Bye