The hot water heater I am aiming to control is a small 20liter unit mounted under the kitchen sink.

It is a plug in unit - it draws only 2.4kw (10A at 240V) and plugs into the same circuit as the dishwasher and a couple of other power points so switching the circuit from the fuse box is not possible.

This controler will connect in between the hotwater heater and the mains power socket and so needs to wirelessly connect to the energy meter.

The Node MCU board containing an ESP8266 withits built in wifi module appears to be the ticket for making the connection and with plenty of IO should happily handle driving the switch to turn the hotwater heater on and off.

It will need to connect to my network, access the energy meter, read the HTML data suplied by the meter, find the energy available data and drive the hotwater heater accordingly.

To simplify the prgraming of the NODEMCU I use the Arduino environment with the ESP8266 Board manager https://github.com/esp8266/Arduino/