How to increase your battery life!
1) Switch to Power Saver Mode – Tap the battery icon on the Taskbar and select ‘Power saver’. If the option is not there, click ‘More power options’
2) Reduce your display brightness – Surface automatically adjusts screen brightness based on the ambient lighting of your environment, but you can always make adjustments yourself. Hit ‘Windows Button + C’ on the keyboard, then Settings, then Screen, then use the slider to adjust brightness. (You can also control Screen Rotation here.)
The Surface Guide 