>All-day battery life is based on 18 hours with the following use: 90 time checks, 90 notifications, 45 minutes of app use, and a 60-minute workout with music playback from Apple Watch via Bluetooth, over the course of 18 hours. Apple Watch Series 5 (GPS) usage includes connection to iPhone via Bluetooth during the entire 18-hour test. Apple Watch Series 5 (GPS + Cellular) usage includes a total of 4 hours of LTE connection and 14 hours of connection to iPhone via Bluetooth over the course of 18 hours. Testing conducted by Apple in August 2019 using preproduction Apple Watch Series 5 (GPS) and Apple Watch Series 5 (GPS + Cellular), each paired with an iPhone; all devices tested with prerelease software. Battery life varies by use, configuration, cellular network, signal strength, and many other factors; actual results will vary.
From "Apple Watch Series 5 Battery Information" (https://www.apple.com/watch/battery/)