While Apple promises that most users should be able to get at least a day of usage out of the Apple Watch, real world mileage may vary. Fortunately, Apple had the foresight to include a “Power Reserve” mode to prevent you from being completely stranded without a watch—you’ll at least be able to tell what time it is, even if you can’t use the other features of your Apple Watch.
Your Apple Watch will start offering to put itself into Power Reserve mode when your battery drops below about ten percent, and will kick into this mode automatically if the battery drops to a level where it can no longer sustain the Apple Watch in full power mode. However, if you know you’re not going to need the more advanced features of the Apple Watch for a while and want to save power for later, you can kick in Power Reserve mode manually either by enabling it from the Battery Glance that can be found by swiping up on the clock face, or by holding down the side button and dragging the Power Reserve slider to the right. In Power Reserve mode, your Apple Watch is basically powered down, but like an old-school LED watch it will still display the time for six seconds if you press the side button. To get out of Power Reserve mode—assuming you have enough battery—simply hold down the side button for a few seconds until the Apple logo appears indicating the Apple Watch is powering up again.