Less is more
The first stories of the iPhone XS battery cases all immediately looked at the mAh capacity of the battery. I assumed Apple put a smaller battery in the case because the battery in the phone was larger, thus needing less weight in the case to more than make up the total charge time. Turns out, I was wrong.
Apple cases have the lowest mAh rating, at 1,369mAh. But that’s because instead of using a single large cell (like the iPhone 7 battery case’s 2,365mAh battery or the iPhone 6 case’s 1,877mAh), Apple is using two battery cells wired in series, which keeps the net amperage the same but lets it boost the voltage to 8.7V, dramatically higher than the estimated 3.8V that the older cases output power at.
Shannon Liao and Chaim Gartenberg go on to say Wh is the correct method of measuring these batteries, which tops out over 10Wh versus 7.13Wh of the iPhone 6 battery case
Source: Apple’s new battery cases are its most powerful ones yet – The Verge