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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.
The Apple Watch supports Apple Pay, too, although we're still waiting for that to come to the UK. A new Apple Watch app in iOS 8.2 (available to download today) lets you see and download apps. Apple also confirmed there would be all-day battery life of around 18 hours.