Looks like ESP8266 will run down to about 2.5v so it's not too bad, but you'll still get better battery life with a good switching power supply.
@Gordon started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Looks like ESP8266 will run down to about 2.5v so it's not too bad, but you'll still get better battery life with a good switching power supply.