Inside the Nexus 6P is a 13.28 Wh (3,450 mAh at 3.85 V) lithium-polymer non-removable battery. The size of this battery is quite good for this class of device: the Galaxy Note 5, for example, includes a mere 11.55 Wh (3,000 mAh) battery with a similar size and type of display, while the 5.7-inch Moto X Style also includes an 11.55 Wh unit.
These non-Nexus large-screened devices don't include Google's latest battery-saving features in Android either, which will likely have a minor impact on battery life, especially in standby. The Snapdragon 810 is known to be a power-hungry SoC, though, which makes the large battery in the Nexus 6P important to achieving decent battery life.
The Nexus 6P doesn't feature amazing battery life, finishing up in the middle of our graphs. However, it consistently outperforms the smaller and less powerful Nexus 5X, and crushes the similarly-sized Moto X Style, which are two good signs for this larger flagship device.
While I don't have full battery life figures for the Samsung Galaxy Note 5, I expect both devices will feature similar battery life, even though the Note 5 contains a physically smaller battery. Samsung has managed to optimize their Exynos SoC well, especially in comparison to the Snapdragon 810, which gives them an advantage in some tests.