This project is mounted behind a solar panel which charges batteries that power the ESP8266 that hosts this project.
The esp8266 in combination with a homemade fz0430 (voltage detector) measures the remaining battery and posts it here.
The video (not mine!) explains the settings of a similar system I am using. For more info, go to http://openenergymonitor.org/emon/
Other sites where I publish live data: http://daturach.github.io/