How to add WiFi to emonTx

Welcome to the OEM forum.

You can either add it on the outside, as the original prototype (that’s before provision was made on the PCB of the emonTx to mount it in place of the battery holder), or if your emonTx has the second FTDI connector in the centre of the PCB, then you can have it inside the case.

The original instructions, including the software download, are here:

CAREFULLY NOTE that the emonTx sends its data on the Rx pin (i.e. the pin marked “Rx” is looking for the Rx pin on the ESP) and it must not receive any data from the ESP. If it does, unwanted characters sent at startup can reconfigure the emonTx in an unpredictable way.
You will need a programmer (a) to load the ESP software and (b) if you need to change the sketch.

There are several issues here.
You will probably need high power rechargeable cells if you do try to operate on battery power, for permanent installation we recommend a 5 V USB supply, because battery life would be extremely short, and the a.c. adapter cannot provide sufficient current without seriously distorting the wave you are tying to measure (so it is current-limited and the emonTx will crash if you try it).
If you want “voltage-less”, you cannot use the 3-phase sketch, because it phase locks to the a.c. voltage in order to measure real power across the three phases. You must use the standard single-phase sketch and measure only current, and allow the sketch to calculate a ‘best guess’ apparent power based on an assumed voltage.