So I wanted to control the power of my Immersion Heater and match it with my Solar Export.
There are pre built solutions solutions for £200+
SONOFF POW R2
Sonoff Pow R2 controls the the whole system ON/OFF with running Tasmota, PulseTime on relays 2 and 3 set to 0.3s
Use a 2.5mm TRRS to connect the POW R2 to the Relay module.
Use the relay outputs across the Up/Down switches on the Power Controller to simulate a button press.
By toggling the Increase button on I can Incease the power by 1% and same for Decrease.
I set automations to monitor the power being imported/exported from the grid and incease/decrease accordingly.
This setup should work for an resistive heater.