Matt Birchler, writing about his Low Power Mode testing:
while most iOS 9 reviews covered this mode briefly and determined it worked as advertised, I wondered what would happen if you used Low Power Mode all the time. I was surprised that no reviewer seems to have done this, so I took it upon myself to give it a try. I don’t have any standardized battery tests that I can do, so I simply spent the last 2 weeks alternating between using Low Power Mode all day, and not using it at all and comparing the differences. My findings are rather remarkable.
I started using Low Power Mode (almost) full time shortly after I installed iOS 9 on my iPhone 5s. The device was about two years old and the battery was starting to show its age — with iOS 8.4.1, the only way I could get through the day was to keep it in Airplane Mode while it was in my pocket. But Low Power Mode changed all that. And while I didn’t track my iPhone’s battery life throughout the day like Matt did, I can tell you that I was able to get through the whole day without a hitch.