question

HI @tridex

I do something similar but way more basic and without monitoring, my water heater is just fixed load immersion heater. I turn it on and off using node red, with a smart shelly switch (with local HTTP api that can be activated via the HTTP node in node red). Similar to your setup but without the complication of variable load water heat and no monitoring.

I would do as you suggest and have criteria for on/off/amount to turn on be controlled by node red running on cerbo and send the load you want turned on to an arduino or similar which outputs the variable voltage to get the various heater power. And have the calculations for how much to turn on/off be done on node red which has access to victron data, and send to the arduino by some form of local API.

This gives most flexibility in terms of data you can use and criteria to base it on.

In terms of monitoring power usage in victron, you could spoof it. Make a virtual energy meter using custom driver (there's some examples on Victron's github page). And calculate the power being used by the water heater?

ie if node red has told arduino to turn on 50% heating power based on the criteria you define and you know what 50% heating power is equal to in terms of watts. Then have node red update the virtual energy meter to show 600w. This is less accurate than real world consumption but probably easiest especially if software is your area!

I think in VRM the water heat usage wouldn't appear on main dashboard (i don't think ac load energy meters do) but the data will be accessible in advanced page of VRM.

You could try installing an actual energy meter (et112) in between the water heater Controller and the resistive water heater to get the real usage data to monitor the load (assuming it's AC power form controller to the resistive heater). This seems more destructive/risky to me.

This might be obvious to you but a mistake i made with my more simple setup:

With regards to speed of turning on and off the heater is make sure you add in some buffers to the turn on/off criteria (i didnt do this originally). You can end up in a loop of the water heater turning on and off if there's not a large enough buffer on the on/off criteria.

ie excess power => turn on water heater => load turns so now there's no excess power

=> load turns off as no excess power

and loop repeats.

just make sure your on/off criteria has some buffers in to prevent it constantly turning itself on and off.

EDIT: All the above is just my best guess at a solution, I'm far from a pro so pleases ignore if not useful.

Whilst the variable load is a nice idea, It's not really necessary, as the hot water heater /tank has a relatively large 'inertia'. Best way to control this would be to relay switch the heater in 10 minute intervals, commencing once the SOC is >95%, and from about 12:00 to 14:00 or peak solar hours. Whilst this may cause a little more battery cycling at the top end, if the solar system power is greater than the heater power plus other loads, this will be minimal.