My Instinct 2 Solar battery life does not make sense

Something to keep in mind is that the state of charge of the battery (indicated in percentage) and what the watch can do with that charge at any given time are two vastly different things.

In the simpler case of a non-solar watch, the percentage will keep going down as the battery discharge, but how many days the watch will operate can and will fluctuate depending on how the processor in the watch averages the power consumption and compares it with the charge level of the battery.

This applies to any battery operated devices, it is just that not many show an estimate of how many days the battery is estimated to last. I have an app in my phone that checks battery level voltage, charge in percentage and estimates hours/days of operation. Right after I turn it on in the morning (I typically charge it overnight, keeping it off) the estimate will be something like 18 hours, because due to the phone booting, email download, notification updates and what not, the phone estimates battery life based on the activity recorded in those first few minutes.

Once I put the phone in my pocket and only check it occasionally, the estimate can jump even to two days, depending on how long it stays idle, despite the battery percentage steadily going down. If I get a phone call or use it for a while to do web browsing or worse to play a game, then the estimate drops easily to even 4-5 hours. But that is always because the app keep updating the estimate based on the current being drawn out of the battery. Once I stop a power intensive activity and leave the phone idle for a while, the estimate rises up again, even if by then the battery percentage is down to 50% or less.

With a solar powered device everything gets more complicated, as the battery is being recharged when there is enough solar/light intensity, That makes the correlation between battery level in percentage and estimated hours/days of operation even more loose.

On top of that, over time the battery will lose capacity due to ageing. Even when the charge percentage is indicated at 100%, it will not be 100% of the original capacity, but 100% of whatever capacity is left, which will be slowly less and less month after month.

With a watch like the Instinct Solar series, in my opinion the best thing to do is to establish a baseline at your actual conditions of use. That includes what sensors you want to keep active, how many activities with or without GPS you have per day/week, what power saving settings you can leave with, how many hours you spend in sunlight, etc. That will give you a more realistic estimate of how long you can go between charges or if given your usage pattern you will be a lucky one and can rely entirely on solar to keep the battery topped off once you give it the first full charge through the USB cord.

Comparing battery life with others or even with Garmin's spec is very difficult and can easily drive you crazy . Yes, I do understand that one may at least want to be sure they did not get a defective watch. The only way to do that is probably to set the watch at the conditions indicated by Garmin as those used for the stated battery life and see if you come close. I would not worry about a variation of few days.