question

How did you go with tis? I'm setting a similar system. I'm using either Load Output or Relay of the MPPT to initiate the switch over. I'm experimenting with feeding PV DC current direct to heater element, currently via a MOSFET.

fHey folks, judging by the comments I'm far less technical than you guys but my planned solution to the above (with limitations) is below. Bit of background, for me goals are ease of install and ease of removal if i move house. I also dont really want a system so DIY/bespoke that a local plumber will panic if he ever comes to service the hot water system.

Use Node-red and a raspi with venus os.

Node red sudo code:

if (battery SOC > defined limit) & (solar power > 3kw) //(mine was going to be 90% SOC).

Send http get request to smart switch connected to immersion heater. My prefered smart switch being from the brand shelly as allows for physical switching and smart.

The pros of the solution.

Relatively easy setup and install with off the shelf parts if you already have an immersion heater. Few lines of code in node red and wiring one smart switch into immersion heater assuming you have on already installed.

Limitations:

• 16a is the highest off the shelf smart light switch I've seen so not appropriate for larger immersion heaters. Although i imagine for some folk making a higher rated relay controlled via http get requests is easy but for me the effort vs gain isnt worth it give my immersion heater is rated for less than 16a anyway

• The above is an inefficient use of power in the sense that DC solar is inverted to AC and then back to DC in immersion heater element. For me this isn't too big a deal because my inverter is in a shed 90m from hot water heater, and i have no intensions of trenching new DC wires in just for heating.