There's nothing too crazy about the battery in the HTC U11: it uses a 3,000 mAh (11.55 Wh) non-removable lithium-ion cell, which is pretty similar to most other flagships on the market. It's perhaps a little to small for the 5.5-inch display, at least in my opinion, though it's not too small.
As you can see from these results, the HTC U11 isn't a particularly fantastic performer when it comes to battery life. In some cases we are seeing a regression from the HTC 10, at other times an improvement, but the phone never pushes into the top echelon of any test.
The good news is that in most tests, the U11 falls around the same marks as phones like the LG G6, Huawei P10 and OnePlus 5, which are in a similar class of flagships. The bad news is that all of these phones are either average or below average in terms of battery life; none make it up to the same levels as the Exynos Galaxy S8 and Xiaomi Mi 6.
In the end it's a bit of a disappointing result from the U11 considering its strengths in other areas, though it's not alone in producing disappointing results for a high-end product.