When you're backpacking in the mountains, deserts, forests, or other types of terrain you could be trekking in, there are no power plugs to recharge any of your portable electronics (GPS, headlamps, water purifier, cellphones, etc.). There used to be only two choices for portable power generation/storage, small solar panels and compact batteries.
Rechargeable Battery Packs: Holds a charge that can be transferred to portable electronics when needed.
Solar Panels: Allows you to convert sunlight into small amounts of energy that can be used for charging electronics.
Power Trekk: A portable hydrogen-fuel celled generator that has several interchangeable heads for powering different portable electronics. All you need to do is add water (you also need to replace the POWERTREKK Pucks when they run out of fuel).