NodeMCU ESP12 Voltage
I edited your question and changed ESP-12 to NodeMCU, based on the title and tag. 

5.1 V will 'fry' a plain ESP12, but the NodeMCU is development board with esp8266 and the board has a power regulator for USB power or Vin pin. The parameters of the regulator depend on the regulator used by the manufacturer of your board.