Yep... You should decide what accuracy you need, and what conditions are. So very shortly:

1. your method: power=(Vpeak*Ipeak*Cos(angle))/2 is realy posyble, when you now, that your signal is 100% sinus or cosinus. Unfortunatly it is imposyble in a real world!

2. You have said, that you are going to use uC with build in ADC. If your controller is 8 bit, expect error of ~10%. May be it sounds a little strange, but it is true - ADC on the uC is realy noisy one, and you will have big error in weak current case!

3. what is dynamic diapazon - if you have very big one - for example from 10mA to 10A, so you need not less then 16 bit ADC, and very good one!

4. How to calculate power? Very simple for active. But is difficult to write there... So look in www.analog.com They have a lot of literature on that, and a lot of dedicated chips for energy measurment. Those chps realy good for simple meters, but too simple for advanced ones!