And Espressif got their act together this time and are working on a good and open-source SDK (ESP-IDF).
I just tried it out this week to make battery powered wireless temperature and humidity sensors. Sleep current for the whole system is not that great (~80uA of which 40-50 are used by ESP32 module). However, the simplicity and ease of use outweigh having to charge the battery every 3 months (3Ah li-ion cell).
Now that you're asking- I don't really have an answer what exactly remains powered on. I have seen reports of people achieving sub-10uA sleep currents, so my result could be improved by a factor of 2 (LDO I put on board has Iq of ~40uA).
I haven't looked really deep at what power modes are available for each power/clock domain- at the moment I just have the ESP waking up from RTC timer every 15 minutes and it can also be woken up manually with GPIO. It's entirely possible that I have left some peripherals at non-optimal state.
I see. I haven't explored the low-power states yet, the datasheet says 5µA is possible using hibernation and timer-controlled periodic wakeup. Was curious if those numbers are achievable.
On the ESP8266 I was able to get down to 16.3 µA deep sleep current.
A lot of easy-to-use modules come with a USB-serial converter and a 5v-to-3.3v converter. These will draw a few microamps even when there's no USB connected - and depending on the precise hardware, some will draw far more than that.
You should plan on buying a few different modules and de-soldering a few components if you want to achieve the absolute minimum current consumption :)
I just tried it out this week to make battery powered wireless temperature and humidity sensors. Sleep current for the whole system is not that great (~80uA of which 40-50 are used by ESP32 module). However, the simplicity and ease of use outweigh having to charge the battery every 3 months (3Ah li-ion cell).