Solar system power for esp8266 device like weather sensor. You need 5V 0.15W solar panel and small li-po battery for example 200mAh. Zener diode stabilizes voltage on 4.3, schottky diode give 0.15-0.2V voltage drop because 4.3V is too much for li-ion battery and prevents lekage current from battery. You can use panel also like binary(0/1) light sensor.
Keep It Simple Stupid
Not tested yet.