So we are looking at this, https://www.ebay.co.uk/itm/203826484109?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=gTu9OIukRj2&sssrc=2349624&ssuid=LqJxcZC1QQu&var=&widget_ver=artemis&media=COPY
Seems to have a fairly linear power generation.
We have 3 immersions on the tank, 2*1kw, and a single 3kw. Looking at a line voltage of 48v DC. So from some highly fag packet based maths about 5kw of immersion should get me the resistive load I need.