It was consistently around 4-5% sitting idle whilst I was asleep. Now it’s 1-2% at the most.
Wow. 1-2% is rather normal for a new iPhone on its original iOS version, 4-5% is beyond awful.
I am not sure how iOS 26 iPhones (the 17 series) fare in terms of standby (I only have your result and it changed with 26.6), but as I mentioned a couple of times, my iPhone Xʀ on iOS 12 and my iPad Air 5 are very poor, with the iPhone Xʀ losing about 6% every 24 hours (maybe the iPad is a little better), but when I upgraded to the 16 Plus and the 11th-gen iPad on iOS/iPadOS 18, the difference was massive.
They are far better, barely dropping on standby and getting something that is a little closer to the golden days of iOS’ standby days.
I have absolutely no idea why this happened. iOS 9 on my 6s and 9.7-inch iPad Pro (when Apple allowed me to run it) was marvellous, matching the battery life of early 32-bit iPads, they just didn’t drop.
When I upgraded I found myself using quite annoyed honestly. Fully optimised settings and you drop 6% per day? And I could never fix it. I was very curious to see what would happen when I upgraded again, and when it happened last year, I was very pleased.
I have seen quite a lot of bashing with 26 when it comes to this, but mostly due to older devices being killed. This, as you can imagine, cannot be compared with my devices, as they all run original iOS versions.
Good to see that at least for you, iOS 26.6 gives some sort of good standby battery life, as it should be.