The 16F628A is a very poor choice for a power meter as it has no ADC on board. The design Tanveer links to will work fine or can easily be adapted to use an 8-pin PIC such as the 12F683 with hardly any code changes.
I want to use PIC12F629, PIC12F675, PIC16F628, PIC16F876 or PIC16F877 just because a have all at hand. In the link suggested by tanveerriaz the circuit looks very accurate and easy to build but the information is not in Watts so will need some math to display the way I want, to me is enought to know the power of my FM Broadcast linear amplifier in Watts up to 100W with one decimal point no other info is necessary.