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The ZenBook 3 packs a relatively small 40 Wh battery, which is slightly smaller than the battery in the MacBook, but larger than the 38 Wh cell in the HP Spectre. Thanks to some efficiency improvements in Kaby Lake, the ZenBook 3 should last longer than an equivalent Skylake laptop, however the small battery size still restricts the total battery life available.
Throughout my time with the ZenBook I typically achieved six hours of continuous use, performing light tasks like web browsing, a bit of game streaming, video watching, and document editing. This isn't a stellar result for the ZenBook 3, and I typically prefer laptops to hit at least eight hours of moderate usage, however the battery did outperform in the HP Spectre in similar circumstances.
Thanks to the efficiency improvements in Kaby Lake's video encoding and decoding block, the ZenBook 3 outperforms most of its ultraportable Skylake competition in video playback benchmarks. The ZenBook 3 doesn't fare so well while web browsing, and a larger laptop like the Surface Book will provide much better results here.
The good news is that it takes less than two hours to fully charge the ZenBook 3 from a drained battery, thanks to Asus' fast charging implementation. The company claims you can get a 60% charge in 49 minutes, and in my testing that figure is close to accurate. So while the battery life isn't necessarily fantastic, and falls short of Asus' claimed 9 hours of usage, you'll be able to quickly charge the laptop on the go.