This 38.2 Wh, 7.5 V battery is rated at 5087 mAh—a 9 percent decrease from the 5547 mAh battery in the Surface Pro 3.

Despite the smaller battery capacity, the 4 generally outperforms the 3 in terms of battery life.

Just to compare Apples to oranges, the iPad Air 2 weighs in with a 27.62 Wh battery, however the upcoming iPad Pro is probably a more apt comparison, function-wise. We'll have to wait and see!