“The easiest way to save power on an ESP8266 is in fact to switch it OFF when not needed and Switch it ON when needed. A typical example of that is a notifier that mail has been delivered or a door has been opened. Opening a mailbox or opening/closing a door activates a Switch that connects the Vcc to 3V3 and the ESP8266 boots up and does its thing. This seems ideal for the ESP8266-01 that does not have gpio16 broken out and therefore is hard to use in regular deep sleep.
This concept will not work though if the switch prematurely is deactivated and the ESP8266 has not finished its job yet. You need some way to keep the ESP running til it’s job is finished and this means it needs to maintain power and thus the regular ‘deep sleep’ seems the only option.
In deep sleep the current consumption is about 20uA. That is not much but it can be brought down even further to 3uA. For this we do not use the regular deep sleep but we will power down the ESP8266 by pulling the CH_PD (=chip power down) pin LOW to Power it down whereas a Switch (or a HIGH output from a sensor) will Power the ESP8266 up and a gpio pin is used to keep it powered up as long as necessary. This is according to an idea by ‘barnabybear’.”