Finding a place to charge a dying phone battery can be frustrating, especially when you’re walking around in the woods for days at a time. That’s why Carnegie Mellon engineers Matt Stanton and Hahna Alexander created Sole Power.

Stanton, an avid backpacker and outdoor enthusiast, spent three years working with Alexander to devise an insole that harnesses the energy generated from each step its wearer takes and converts that energy into electricity which it stores in a battery. Earlier this month, the invention earned the team a Popular Science Invention Award.

The insole weighs less than five ounces and can be used to charge electronics through a USB port. While the current model, due out commercially later this year, is able to fully charge a smartphone with a 15-mile walk, future models will be able to do it in less than five miles.