If you're already using rechargeable batteries a solar powered battery charger might make a lot of sense.
You can power those batteries up with sunlight instead of plugging into the electrical grid.
There are two ways you can go:
If you already have a suitable solar panel your best bet would probably be buying the battery charger with CLA. This also gives you more flexibility since you can put the panels in the sun and have the charger set somewhere else.
Otherwise there are a wide range of self-contained solar panel/charger units available. Many are "universal" chargers that allow you to charge AAA, AA, C, and D batteries. Some also come with adapters that will let you charge up your ipod or cell phone too.
Charge times vary depending on number of batteries and how direct the sunlight is. Look at 2-6 hours as a ballpark figure.