... what i need to construct now is actually a frequency counter? Which means i get the output of VFC (is a pulse with frequency 10-10kHz) and display it with a frequency counter is it? Just want to double confirm is my idea correct?
Sure! The 4521N chip in the left bottom corner of the above schematic creates the time base for the power measurement (various time units accessible thru Q18 .. Q24, s. the 4521 data sheet). For energy measurement disable the time base unit and simply count up (until overflow ;-) ).