I have pretty much the same setup, with an immerSUN unit (the predecessor to the Eddi) driving an immersion heater in the DHW cylinder from any excess solar generation. This handles the majority of my DHW demand and I find it takes the cylinder to 60 degrees at least once a week - even in winter.
I do not include this immersion heater in the stats I report to HeatpumpMonitor (though I do have it metered so I could add it) - the rationale being as Tim has outlined, that it’s unrelated to the performance of the heat pump.
This question was noted in a post from Trystan a while back - which is quite a long post so I’ll quote the relevant bit here:
A slightly tricky one is how to deal with solar PV divert to an immersion heater? Should that be included in the system boundary of the heat pump? It seems like an unfair penalty for something that makes total sense economically especially if you dont have a battery to be used as a buffer for running the heat pump to do DHW in the summer…
There is an effect on the reported SPF from excluding the immersion, since the heat pump tends to run fewer DHW cycles (which are typically at a lower CoP than heating cycles) so the SPF will tend to be higher than for a system without solar PV diversion. However, including the immersion (at a CoP of 1.0) would be more misleading, in my opinion.
On my system’s summary page I have ticked the box to say it “Includes booster & immersion heater” - on the basis that the electricity consumption recorded for the heat pump does include the auxiliary heater within the main heat pump unit - which can be brought in to ‘help’ the compressor if required. @Timbones - is that the right thing to do in such a case, so should Andrew tick that box too (even though the immersion in the DHW cylinder is excluded)?