Apple today announced that watchOS 9 will feature a new Low Power Mode on the Apple Watch Series 4 and newer. When the Low Power Mode is enabled, the Apple Watch will be able to get up to 36 hours of battery life per charge.
When the Apple Watch is in Low Power Mode, many core features will remain available, including activity tracking and fall detection, but some features like the always-on display and workout detection will be disabled to reduce power consumption.