How accurate you are going to aim? There are short time frequency and phase errors on the line frequency. Also you need a switch to select the mains frequency, 50/60 Hz. You should also ensure that the PIC clock is TXCO giving a good stability. If you just want an (approximate) idea of the power consumed, then you can assume the line voltage is a sine wave and the load current is also a sine wave. Then you will get the power consumed at the fundamental frequency. Measuring phase accurately with a 8 bit accuracy is not easy because of the conversions involved (sine and cosine functions). If you want to include harmonics in the overall calculation, you will need to average over 2-3-4-5 cycles and also sample over 1/2, 1/3, 1/4, 1/5 the line period (20 ms for 50Hz). As KlausST has pointed out, you also need to take care of the Nyquist theorem.
It will be a fun project to do.