To clarify, this is an integrated rechargeable lithium-polymer battery of 28.65 Wh (https://www.apple.com/chfr...). And indeed, it is excellent. Note that it is also used to recharge the Pencil (which can go down quite quickly), but the impact seems to be relatively small.

i don't know exactly the capacity but i have to say that the battery is really good! Can last easily 2 days with heavy work!