One of the major issues of Apple's new line of Watch smartwatches seems to be the low battery life. The firm admits the device has a battery life of just 18 hours over a "normal day", but admits this can fall to as little as three hours when placing calls from the Apple Watch.
The 18 hours of battery is based on a usage pattern that includes 90 time checks, 90 notifications, 45 minutes of app use and a 30-minute workout with music playback from Apple Watch via Bluetooth. Even if you're only using the device to check the time, five times an hour for 4 seconds each, the Apple Watch's battery should last only about 48 hours although there's a Power Reserve mode that promises to bump this up to 72 hours.
Apple then broke its testing down into six categories: all-day battery life, talk time, audio playback, workout, watch, and power reserve. As mentioned in the keynote, the Apple Watch gets 18 hours of battery life — that was with the following usage patterns: "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." It's hard to say without using the Watch in practice, but that sounds like pretty light-to-moderate usage. 45 minutes of app usage doesn't sound like a lot — but that's coming from a smartphone usage model, not a watch.
The talk time and audio playback tests are pretty self-explanatory: you'll get about 3 hours of battery when placing calls from the Apple Watch, and about 6.5 hours of music playback when paired with an iPhone using Bluetooth. It's possible to play music directly from the Apple Watch, but Apple didn't say how long you'd be able to do that. If you're thinking of using the Apple Watch as a dedicated fitness tracker, you'll get about 7 hours of usage — Apple says that's how long it'll last with the workout session active and the heart rate monitor turned on.