Comparing solar immersion heater diverters (water heating).

[Oldgreybeard]

My gut feeling is that all of the available diverters work OK on their own, and do a good job of utilising excess PV generation to heat hot water (or in our case a thermal battery).

Where some fall over is when a battery storage system is installed, when it's possible (probably even very likely) that the battery system and the hot water diverter will "fight" each other, with the first to respond getting the excess PV power. This is what happened with the Apollo Gem unit I had, it worked fine until I installed the battery. After the battery was installed there was no way that I could reliably get the Apollo Gem to have first dibs at any excess PV generation, almost always the battery inverter "won" and took priority.

I tried adjusting thresholds on both the PV diverter and the battery inverter/charger, but couldn't find a combination of settings that would reliably ensure that the hot water had priority. I opted to swap out the Apollo Gem for an Eddi, only because the Eddi had the option of adding additional CTs, and with a CT on the feed to the battery system it was easy to set it up so that the Eddi always took priority.

The rationale behind wanting the Eddi to always take priority is that, in winter, it's always better to heat the hot water from PV first, rather than charge the battery first.