Re:Putting PIC18F to sleep for a long time
"Counting the WDT interrups is also doable but, I really need to see what is the impact on consumption. "
I think if you do the math, you will find that the periodic wakeups are insignificant to your total power consumption.
For example, if you wake up for 20 useconds every 10 seconds, then you have a ratio of (20e-6/10). That's a 99.9998% of time spent in sleep. Will you really save a significant amount of power by striving for 100%?
By the way, remember that the WDT is RC controlled, so it is not accurate in time and can vary significantly with temperature.