I've had a devil of a time trying to figure out whether Windows 10 is helping or hurting when it comes to battery life. The answer: It's complicated.
This entire week, we've been measuring battery life across four different Windows laptops: two versions of the superlative Dell XPS 13, one excellent HP Spectre x360, and my very own Lenovo ThinkPad X240. This entire week, I've been running battery life tests almost every night, along with real-world tests during the day, to figure out how long they last. We tested Windows 8.1 against Windows 10.
The reason we've had to do it almost every night is that the numbers haven't been consistent. One day, a Windows 10 laptop would give me one number. The next, two hours less.
But I think I figured it out. Windows 10 silently updates itself in the background, and there's next to nothing you can do about it. And when it's updating, I think it's warming up the processor, which drains the battery more quickly.
How much more quickly? Well, my HP Spectre x360 got 6.5 hours of battery life on Windows 8.1. One night, it only managed 5.5 hours on the exact same test. That's a huge dip!
But the next night, after I let the laptop stay on all day (hoping it would take care of its update business early), it managed 7.5 hours. A whole extra hour!
The high-res 3200 x 1800 Dell XPS 13 didn't see a big gain, though. More like breaking even. On Windows 8.1, it managed about 5 hours, and I got about 5 hours with Windows 10.
Our Dell XPS 13 with the 1080p screen, though, saw a definite jump: from 6 hours in Windows 8 to 6.5 hours in Windows 10. Not bad at all.
The ThinkPad was the most surprising result of the bunch: from 8.5 hours in Windows 8 to a full 10 hours in Windows 10, with its giant secondary battery installed. Hell, it was still running when I woke up the next morning.
But I know now, from first-hand experience, that I won't necessarily get those ten hours of battery life on a charge. Just the other day, my ThinkPad barely had enough battery life to get home from San Francisco while writing about a hotly-anticipated new smartphone. And I'm pretty sure it was because Windows 10 was downloading those mandatory updates -- and using up my LTE data on the train -- without giving me a veto option.
I also wouldn't trust the new Battery Saver button to save your butt: in a couple quick anecdotal tests, it didn't add more than about 10 minutes of battery life to the total. You'll want to use the legacy Power Saver mode (buried in advanced power settings) if you want extra longevity.
So yeah, Windows 10 may actually have better battery life! Except when it doesn't.