Chemelec,
Thank you very much, I visited your site and found plenty of very interesant projects, really very good. I saw the circuit diagram for the Watt-Meter, and it is for instant readings; I need to accumulate data during periods of about 20 hours.
Nigel,
Thanks too for your approach, I need to read a current sample, may be each second, then store it in a register and add to another. the next step would be to divide the latter by 3600 (seconds/hour) and display it. Am I right?