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The HTC U Ultra includes a 3,000 mAh (11.55 Wh) non-removable battery, which is very small for a device of this size. The Google Pixel XL, for example, contains a 3,450 mAh battery in a smaller body, while the Huawei Mate 9 loads in a 4,000 mAh battery into its 5.9-inch body. 3,000 mAh is too small for a 5.7-inch handset: it's the same battery capacity as the HTC 10, the company's last flagship with just a 5.2-inch display.
While the Snapdragon 821 is a reasonably efficient SoC, the small battery capacity is not enough for daily usage. Switching over to the HTC U Ultra as my daily driver felt like I was going back to the days of the Galaxy S6 with its poor battery life, and this isn't what I've come to expect in 2016 or 2017.
As you can see in the charts above, the U Ultra is obliterated by phones like the Pixel XL and Mate 9 in nearly every test. Just 4.45 hours in PCMark's battery test is a terrible result, and puts this handset well behind basically every other flagship I've tested in the past 12 months. Those who demand good battery life from their phone should look elsewhere.