Finally the sleep code is up and running. I've set the device to power on every hour and do its full sending cycle to test battery life. Here's what the time span from yesterday afternoon until this morning looks like.

The green line on the bottom indicates whether the battery is charing (10), not charging (0), or full (20).

Weather is pretty bad too, so these are truly ideal test conditions. It seems that per hour, around 2% battery is lost. Next thing to determine is whether (or rather, how much of) this loss comes from the sending process (great result) or idle state (not so great result). This will be tested by gradually increasing the sending interval and comparing results for different intervals.

Another possible issue I've noticed is that solar panel wakes up the sim808 module from time to time due to its reaction to getting power over USB. It could be that the module still chugs down current instead of staying put when we finish sending.