Preview at WWDC likely in June, followed by September launch.
Apple Watch Impacts iPhone Battery Life... One Way or the Other
However, there have been mixed reports about the impact the Apple Watch has on iPhone battery life. The Apple Watch needs to be paired to an iPhone for full functionality. The iPhone connects to the Apple Watch via Bluetooth and Wi-Fi to feed it data and notifications throughout the day.
Some users have found a notable improvement in iPhone battery life with the addition of an Apple Watch. This suggests the act of offloading notifications and quick interactions to your Apple Watch, could make your iPhone battery last much longer.
The best unexpected but /now/ obvious surprise to having the Apple Watch is plenty of battery life on the iPhone after a full day out!— Dan James (@Daniel277) April 26, 2015
However, several other users have noticed a significant drain on their iPhone batteries in early usage with the Apple Watch.
Apple Watch battery life: 16-20 hrs. iPhone paired with Apple Watch: seemingly zero hours— Jacqui Cheng (@ejacqui) April 26, 2015
Former Engadget editor Ryan Block had as similar experience and pinpointed the Apple Watch Companion app as the culprit. John Byrne also puts some blame on the Companion App, saying that force-quitting the app seemed to help at least a bit. A discussion thread in our forums provides similarly mixed results, with one user blaming his battery drain on checking email on the Apple Watch. Due to the inconsistency in reports, it seems that either a software bug or particular usage pattern could be a culprit.