I got a ESP-12 unit, with a breakout board. You can directly solder the LDO on the back of the breakout.
A used Nokia LiPo battery, similar to this. You can use any 3.7V LiPo battery.
A soil humidity sensor.
A LED, some caps and resistors.
The AOUT of the Soil humidity sensor goes into the ADC pin of the ESP-12 for humidity reading.
ESP-12 wake up every 5 mins (not really have to, but just for testing, ideal is every hr), take the reading of the ADC input, and publish the result to thingspeak.
Been running for days, so far so good, did not do any real current measurement. Waiting for my solar panel and relay modules to complete the whole project