Now that you can control your NXT robot with your Android phone, rather than using yet another electronic device, there are ways to save the planet even further, thanks to Dexter Industries’ dSolar Systems. The 9V solar power system allows you to rig your Mindstorms NXT robots for harnessing the power of the sun.
The solar system provides about 250 mA, enough juice to power your NXT robot and a single motor. They also offer capacitor banks, battery pack rechargers and parallel connectors for linking three panels together on one bot. The lone system will set you back $90, quite pricey for such a low power source, but worth it for the compatibility convenience.