Welcome, Graeme to the OEM forum.
My understanding is the Immersun takes slices out of the mains power at very high frequency and feeds a smoothed version of that to the immersion heater. The proportion that gets diverted is continuously variable and the objective is to balance the power fed to the immersion so that the nett power flow at your grid connection is never export.
So after the filter, the immersion (because it’s constant resistance) sees both variable voltage and pro-rata variable current.
If there isn’t enough power being fed into the immersion heater to overcome the losses through the insulation and from usage, then the tank water will cool until there’s a balance. If there is a small excess at whatever temperature, then the water temperature will increase, and so will the losses, until there’s a balance again. If there’s enough excess power to take the temperature up to the thermostat setting, the Immersun will be deprived of its load and the energy will be exported. The Immersun will only import if you ‘boost’ manually.
Do you mean feed-in tariff or export tariff? You get the FIT payments depending on the energy you generate, whether or not you export it. If you don’t get FIT payments, you get an export tariff payment instead. Without knowing the value of that, it’s highly likely that you will be better off keeping as much generated energy as you can.
What I think would happen then is the tank water would cool at the natural rate. I think the tank losses will be proportional to the temperature difference (as most will be conduction and convection rather than radiation), so it might be more economical to not aim to maintain the water at the thermostat setting. Whether that’s acceptable is your decision.
I think the only way to determine this will be to run a series of tests, carefully measure and record all the powers, energies and temperatures and then work out the costs.