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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 30-minute workout with music playback from Apple Watch via Bluetooth, over the course of 18 hours.
Cook had previously hinted that the Apple Watch would need to be charged nightly. That 18 hours of battery life includes checking the time, receiving notifications, using apps, and doing a 30-minute workout. As with any device, battery life will vary depending on your usage.