In short: Boot your laptop into the shell; it uses much less battery.
Sometimes, I could do with my laptop as a plain text editor. Maybe I'm taking notes in class; maybe I don't want to be distracted while I write; maybe I'm doing some writing on the plane.
If you find yourself in a similar situation and you're comfortable with the command line, you can do all your work via command line and save tons of battery.
On a Mac, simply hold C as your computer boots and you'll be given a shell. It's harder to do on other operating systems, sadly, but it can probably be done.