lnp,
For single phase, from a potential trasformer get the voltage sample. from a C.T get current sample.from P.T sample switch on a timer and C.T sample swith off.from this get the power factor. multiply all the three this is real power in KW/WATTS and integrate over a time.This Kwh/Wh/Energy. This is easily said but you do it for 3 phase.
V.D.DASS