@kimot Thank you for your comment & tips.

Thank you for your input. In order to lower power consumption I do not want to wake up ESP8266 every hour. That's why I did not choose the deep sleep route. ESP32 could be use instead for longer deep sleep, but seems to me be overkill for such simple circuit. Sure voltage checking would nice, no doubt. Nevertheless it is a good idea to check the board from time time that it is working correctly.