**The rms value results from squaring the values over intervals of time, finding their average value, and taking the square root of it.**

Thinking of measuring power used by a common appliance connected to 220 V AC monophasic using a PIC for the measurement and calculation, I understand I should do this:

a - Measure V and simultaneously I (well, as close in time as possible to each other!!).

b - Multiply V * I to get instantaneous value of power.

c - Add succesive instantaneous values of power.

d - Calculate the square root of their sum

e - Repeat the process for the next cycle.

My questions:

1 - How many times per full period (or per second) should I measure instantaneous V and I to make this reasonably precise? I could not find a suggested figure in the so many places talking about true RMS.

2 - Is the zero crossing information a must to start counting the above or could I use a precise timer to trigger the next pair of measurements plus the calculation?

2 - Is it wrong to do this kind of measurement/calculation for one of every two full periods? Am I cheating the reasult in that way?

3 - A 10-digits ADC is it enough if taking the output from current / voltage transformers with adequated down scaling?

4 - Even if the above is a yes, could the 18F452 (which I've just started to work with) be useful here? (Quite an improvement over the 16F877!!)