max = 0;
t = millis();
while (1)
{
  instantCurrent = readAnalog();
  if (instantCurrent > max)
    max = instantCurrent;
  if (millis() - t > 1/50) //period is over.
  {                        // prepare for the next maximum in the next period
    Irms = max/sqrt(2);
    AveragePower = 220 * Irms / 50; // --> THAT'S WHAT I WANT
    t = millis();
    max = 0;
  }
}