How to measure ac Power using a micro-controller?
If you want to measure appliance power you need to digitise signals that represent alternating voltage and current waveforms. You should strongly consider simultaneous digital sampling of both quantities at a sampling rate of at least 1000 times per second in order to achieve some measure of accuracy. Multiply each simultaneous pair together and average the resulting waveform to give you power.

Using a dc quantity to represent voltage should not be regarded as having any merit. Trying to calculate RMS values for voltage and current then trying to measure phase difference by zero crossing is naive given the harmonic nature of current waveforms in a lot of appliances. This is not how pros measure power.

Power = voltage x current whether it's dc or ac, analogue or digitally sampled.