I can't understand why a PV diverter needs to know the temperature of anything.  Mine only knows about the power flowing into or out of the house supply and needs nothing more to work perfectly.  Others are similar to the way mine works, as I looked at how a few work before building mine, although I haven't heard of the Apollo GEM before now.  The main variation between the different makes available is how they switch excess power to a load (and it can be any load, doesn't have to be a water hea