https://techcrunch.com/2022/09/23/a-guide-to-low-power-mode-on-the-apple-watch/

Battery life has long been the Apple Watch’s biggest drawback. The issue only became more pronounced when the company went all-in on sleep-tracking with watchOS 7 in 2020. Rather than increasing the mAh on the hardware, Apple presented a workaround: an alert that pops up if the user dips below a certain percentage before bed time, requiring a recharge.

The Watch Ultra, which goes on sale today, finds Apple addressing the problem head-on. The larger case (49mm to the Series 8’s 45), coupled with a slimming down of components like the haptic motor, offer more internal space for the battery. As a result, the sports-focused model effectively doubles the Series 8’s 18 hours to 36, in spite of a slightly larger screen and double the brightness (from 1,000 to 2,000 nits).