Immersion diverter using arduino and cheap commercial dimmer

This diverter is easy to implement using an arduino and an £8 dimmer
such as Dimmer module for 16/24A 600V High Load, 1 Channel, 3.3V/5V logic|Integrated Circuits| - AliExpress
The arduino is connected by a usb cable to an emonPi and receives the excess power
emon/emontx1/power1 over the serial usb link from node red running on the emonPi.

I have some addons that read the temperature of the tank so as not to overheat, and switch the heater on in the off-peak period in the night without regard to the excess solar power.

I have bonded on some extra heatsink to the dimmer.

Have you thought about filtering, and the rules concerning the generation of harmonics and the limits for harmonics injected into the public electricity supply?

How would I measure these, please?

You would need to either hire the specialist equipment needed, of take your device to a test lab that has the necessary equipment and expertise to carry out the tests for you.