For power line Enerzy measurement a excellent single chip solution from cirrus in form of CS5460 is available.(www.cirrus.com)
This chip require just few resistance & other component for sensing voltage & current values.
Chip samples voltage & current values at high speed (sigma Delta Convert) & using DSP filters & algorithem computes powers
(P = In * Vn + In-1 * Vn-1 + In-2 * Vn-2 +....).
Sampling frequency for In & Vn is in KHz(4 KHz) range.
This algori takes care of power facors as well.

In built DSP filters & algor takes care of noise also.

CS5460 is avavilable is India at cost of Rs 250/- ($4.0). Cirrus site has full application notes which list circuits( with compo values) for all types of single phase enerzy measurement. Serial interface
(SDI, SDO , SCK)is available for interfacing with microcontroller

Microchip web site list complete Single phase enerzy measurement project for their micos using CS5460