From Gizmodo’s coverage of the Roost, 9V battery for your fire alarm that’s connected to your wifi network for instant push notifications if the smoke alarm goes off:
Battery life is rated at an impressive five years, even while the Roost is connected to your home’s Wi-Fi network. But that’s because the Roost pretty much sits completely dormant until it detects your smoke detector’s deafening screech. At that point the Roost springs to life, connecting to your home’s Wi-Fi network in order to send notifications to your smartphone.
Brilliant. Here’s the Roost’s website for some more information.