The current RPi A is about 2.5W, so already pretty cheap to solar power w a battery/small panel. Search for "solar rpi" etc to find lots of examples. Depending

The current RPi A is about 2.5W, so already pretty cheap to solar power w a battery/small panel. Search for "solar rpi" etc to find lots of examples. Depending on your use case, the Beaglebone Black is a good alternative and idles at about 1W.