I've seen a few questions of the forum about how to use excess power to heat water. Here's my plan (img) for a fully off-grid installation. I'm using a microcontrolIer to nuance the logic (and happy to share that code). I could use the relay out of a Cerbo for simplicity but I'm using the GX in the EasySolarII so no relay available. BMV relay also a nice option.
I'm using a cheap (arduino-based) microcontroller to read for sufficient SoC and PV Power (and possibly Charge State and Battery Power) to then drive a logic pin tricking the temperature differential controller into thinking it has heat in a liquid solar collector. The TDC drives a beefy MOSFET into conduction if heating is in demand. Note, there's no additional AC load impacting the inverter twins, this is purely "opportunistic" 50VDC load.
You could bypass the TDC and have the microcontroller read temps and drive the transistor, but I'm using the TDC to also shunt heat out of a wood stove during winter and it needs to manage both sources. The design presented here helps in the summer months, when there's excess solar and less wood heat. The logic is described in the image.
The Tank140 logs the state changes for me so that I can view the activity remotely. I plan to add a second BMV to track the diverted power. The TDC also has a remote dashboard that tells the whole story. I tend to idle the water at 30degC then ramp it to 55degC some hours before needed.
