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The Arduino microcontroller is finding its way into labs throughout undergraduate physics curricula, from introductory courses to a variety of beyond-the-first-year laboratory classes. At Davidson College, we use Arduinos in a gateway STEM course for students who are interested in energy and the environment. Students learn to build simple circuits and write the accompanying Arduino code to control the temperature in solar-powered model buildings. To make the models fully solar powered, the Arduino itself must be powered by the Sun—no batteries allowed! Hence, we replace the 9-V battery that is usually used to power an Arduino with a 13.5 cm × 12.5 cm 9-V solar panel (DFRobot part #FIT0330), which can generate a maximum short-circuit current of approximately 200 mA. We find that the solar panel works well for most tasks, including temperature measurements, liquid-crystal display (LCD) illumination, and SD card module operation, but cannot generate enough power to drive a servo motor, which needs several hundred milliamps. For these situations, a 9-V, 1-F capacitor connected in parallel with the solar panel can store energy during the rest period between brief high-current operations and supplement the solar panel when higher power is required. [ABSTRACT FROM AUTHOR]