Apple claims that iOS 9 gives you an extra hour of battery life, but you can eek out even more using the new “low power mode”, which can temporarily disable a lot of power hungry features such as email fetching, background app refresh, automatic downloads, and it even slows down the processor and network. Some people are finding that it gives you about double the battery life.
Your phone will prompt you to enter low power mode when your battery reaches 20% and 10%, but if you know you’ve got a long day ahead you can proactively enable it by visiting Settings > Battery, and sliding the switch to turn on “Low Power Mode”. The battery icon turns yellow to remind you.
Just remember that your phone can’t alert you of incoming emails since your iPhone will only check when you manually open the Mail app. And there seems to be no way to permanently enable Low Power Mode, as it resets after your phone is nearly charged up.