blog.thomasheldt.de/esp8266-wifi-briefkastenmelder-mit-solarmodul-der-start/#comments
forum-raspberrypi.de/forum/thread/19121-raspipo-st-2-der-briefkasten-verschickt-e-mails/
www.thomaslang.nl/esp8266-dht22-lowpower-mqtt-sensor/
//Sleep for 120 seconds, then wakeup and send data again ESP.deepSleep(120 * 1000000, WAKE_RF_DEFAULT);
https://www.open-homeautomation.com/2016/06/12/battery-powered-esp8266-sensor/