The Pixel XL comes with a sizable 3,450 mAh (13.28 Wh) non-removable lithium-ion battery, which is large for a 5.5-inch phone. This is the same rated battery capacity as the Nexus 6P, however the Pixel XL features a smaller display and a more power efficient SoC. Competitors like the OnePlus 3 (3,000 mAh) and the Galaxy S7 Edge (3,600 mAh) fall either side of the Pixel XL's battery capacity.
Immediately you'll notice that the Pixel XL packs much better battery life than the Nexus 6P, and it comfortable beats Snapdragon 820 competitors like the HTC 10, LG G5, Sony Xperia X Performance, and the OnePlus 3.
There are a couple of interesting things to note here. The Pixel XL packs a 15% larger battery than the OnePlus 3 in a phone with the same display size and type, however battery life is more than 20% better. This indicates the Pixel XL is a more efficient smartphone, which bodes well for the Snapdragon 821's claimed power efficiency improvements.
However, the Galaxy S7 Edge still easily beats the Pixel XL in web browsing battery tests. The Galaxy S7 Edge lasts around 18% longer in general from a 4% larger battery, which tells me that Samsung's home grown modem and SoC are more efficient than Qualcomm's at these tasks. The gap is closed significantly in PCMark, though, where SoC and display efficiency are more important.
The Pixel XL features quick charging, however the implementation is slightly slower than the Nexus 6P due to reduced charging speed in the 90 to 100% range. The ability to gain a 60% charge in under an hour places the Pixel XL around the same mark as other flagships, and Google claims a 15 minute (roughly 20%) charge is good for 7 hours of "use".