Apple is quoting 10 hours' battery life on the iPad Pro 12.9, and in my testing I'd say this was probably about right when it comes to movies and web browsing. The battery test we run for all devices is simple: 90 minutes of a Full HD video at full brightness from a full charge and see how we get on.
I was expecting the iPad Pro to fly through this challenge, given it's got a powerful (but presumably efficient) processor powering things at the heart, so great battery life would be a cinch.
However, it was actually almost identical to the iPad Air 2 and iPad mini 4, dropping down to 72% battery in an hour and half. That means you'll probably get six hours of movie watching at full pelt (with full brightness and connections all running) which should improve a lot when pushed to airplane mode and lower / automatic brightness.
When I initially started the test, I was surprised to see the battery dropping considerably quicker than on the iPad Air, which took ages to even drop a single percentage point, where the iPad Pro had lost three percent in the first 10 minutes.
Then I realised I was propping it up using the Smart Keyboard - which draws power and data through the Smart Connector. I took that off, recharged the Pro to full and it was a lot better. Note: don't keep your tablet on the keyboard if you don't need to - perhaps invest in a dedicated cover for the other times (which will lower the weight too - although won't help with the productivity).
The iPad Pro 12.9 curiously doesn't have as good battery life as the mini 4 and Air 2, but it's only three percentage points difference from the test. That's about 12% poorer performance, which I didn't expect when we have such a powerful machine. It must be driving all those extra pixels, but it was an aberration I wasn't expecting.
The general battery usage of the iPad Pro is actually hard to quantify, in terms of comparing it to the other iPads in the range. That's because I've been using tablets for years, and I just find that phones can do nearly everything without needing to decamp to a separate device, so a tablet just faded into the background.
I'd still pull out the Air for watching movies, but without a dedicated data connection on the go, it becomes quite the hindrance. But the iPad Pro is just such a new experience in terms of being able to swiftly switch to emailing with a real keyboard and stylus for easy annotations that I was just using it so much more than any other tablet for years.
This meant that I was burning through the battery at a terrific rate. When doing the review as mentioned above, I started at 9AM and was down to 33% battery when I stopped at 1PM. That's a bad performance on a phone, but then again I wouldn't have that phone in constant use for four hours.
You'll need to charge the iPad Pro every other day at least if you want to keep it in use, as the battery life will diminish quickly if you're constantly tapping away with the Pencil or the Smart Keyboard. It's still a decent performance, and I'd rather have something I'll use all the time.