Apple hasn't revealed the battery capacity for the iPhone 6S Plus - it never does - so it's difficult to say just now how big the power pack inside the device is. The iPhone 6 Plus had a 2915mAh battery and considering the near identical dimensions the smart money is on the same again.
The Cupertino firm has quoted the same battery life statistics for this handset as it did the 6 Plus, which includes up to 24 hours talk time, 14 hours of HD video and up to 80 hours of music.
In real life usage you're unlikely to get close to any of these figures, but I was pleased to find battery performance followed very much in the same footsteps as the 6 Plus.
I regularly got to the end of the day with around 30% left in the tank, and that included emails, calls, texts, social media, games, music streaming and web browsing throughout the day.
Play a game or watch a movie for an extended period of time though and the large 5.5-inch display on the iPhone 6S Plus will start to take a toll on the battery. The relatively basic New Star Soccer managed to gobble away a decent chunk of power, but I suspect most of that was from the screen.
I ran the techradar 90 minute battery test video on the iPhone 6S Plus, with the screen on full brightness and accounts syncing in the background over Wi-Fi. From 100% the battery dropped to 78% by the end of the video, losing 22% of life in the process.
Watch how the iPhone 6S Plus battery compares to the 7 Plus and 6 Plus
That's an improved showing over the 6 Plus which witnessed a drop of 27%, while the iPhone 6 lost 26%. It appears the A9 processor is a little more power efficient than the A8, and the 6S Plus didn't heat up much during the test.
It's curious as the rumors are the battery is smaller, and the iPhone 6S performed poorly in the same test - dropping 30% during the video test.
A new feature which arrived with iOS 9 is Apple's own low power mode - a function which has been present on Android handsets for years now.
This will be music to many Apple fan's ears, as fast discharging iPhones have been a worry for a number of users in recent times.
In a bid to save power the iPhone 6S Plus turns off background mail, the always-on Hey Siri command, automatic downloads and some visual effects in the interface. You can turn the function on by diving into Settings > Battery, although it will prompt you to turn it on when you get below 20% juice left.
You'll know when you've got low power mode switched on as the battery icon will be yellow instead of green. I switched this on a few times during my time with the iPhone 6S Plus and it kept me partying past 4am (after coming off charge at 7am).
It's not as fully formed as Sony's or Samsung's power saving modes, but it's certainly a step in the right direct and any form of power saving will be welcomed by the Apple faithful.