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I would solve the problem with a Sonoff POW R3 reflashed with tasmota. Set the heatpump controller to always be ON and control when it receives power using the Sonoff.

I happen to have a parallel 5kw Multiplus setup like yours which I installed myself.

Well the GRID SETPOINT should not be ZERO. Grid setpoint This sets the point at which power is taken from the grid when the installation is in self-consumption mode. Setting this value slightly above 0W prevents the system from feeding back power to the grid when there is a bit of over-shoot in the regulation. The default value is therefore 50W - but should be set to a higher value on large systems. Do you have a ET112 grid meter giving you accurate results or are you using the measurements in the multiplus units?

Yes, get a geyserwala. It replaces the controller on the wall and allows you to connect to it via a webpage or a app. Cost less than a R1000. Easy diy fit.

My Victron Integration provides a grid power entity. I use it to determine if there is loadshedding. Remember to cater for hysteresis. Add a time condition to your trigger. This automation is currently disabled since it pre-dates my geyserwala setup. The geyserwala simply switches to solar only during loadshedding. The geyserwala+HA setup is simply brilliant.

That will work fine. I often buy from communica and they are legit. Another option if they don't have stock is to get one from Micro Robotics. Robotics.org.za https://www.robotics.org.za/cables/usb-cables/usb-converter/USB-SERIAL

Have you done the setup via VEConfig on the multiplus? If not your low voltage cut-off is still set to 48V. This is too high. Victon uses a low frequency switching design to create AC. This means that there is a big toroidal transformer that needs to charge up when you switch from bypass to on-line mode when the grid falls away. You get a big inrush current that the comparatively small battery can't provide. 1) setup the multi correctly via VEConfig. Use the portal and edit on a computer and re-upload. 2) Add another battery to bring the system in-line with what Victon reccomends should be the minimum battery size for a 8KW inveter.

Thanks Paul, didn't know it changed in the last update. Will have a look at it again. Last time was more than a year ago.

I run two Victron multiplus 2 5kW units in parallel. In the Victron guide for parallel setups they stress the point that your DC cables running from a busbar need to be of equal length. BUT the also state that your AC cables need to be of equal length as well and NOT be over sized. There needs to be a bit of resistance on the AC side for the units to correctly operate in parallel. I know you have 2x Sunsynk units but maybe this can help. To quote: Warning against over-dimensioning the AC wiring Note: Do not over-dimension the AC cabling. Using extra thick cabling has negative side effects. Technical background: for a properly working parallel system, the AC current should be evenly distributed between the paralleled units. The resistance in the cabling helps with that and is needed for that; to overcome small differences between one inverter/charger and another, for example in the AC contact on the AC input. When the resistance in the cabling is too low, such small differences in resistance of the current path in a unit itself can results in a large relative difference. This results in bad current distribution. An exaggerated example: Using 2 units (A and parallel and using too good cabling, one might achieve a total resistance for Unit_A of 0.0001Ω and a total resistance for Unit_B of 0.0002Ω. This results in Unit_A carrying twice as much current as Unit_B. Using the same 2 units in parallel with, for the sake of this example underdimensioned AC cabling one might end up with a total resistance for Unit_A of 15Ω and a total resistance for Unit_B of 16Ω. This results in a much better current distribution (Unit_A will carry 1.066 times more current than Unit_A) even if the absolute difference in resistance is much bigger than in the previous example (1Ω vs 0.0001Ω). A side effect of over dimensioning the AC cabling can be faulty Power Assist operation. Out of all units, the phase master is in control and measuring the AC input current. And in case that current is (grossly) unevenly distributed between the paralleled units, the resulting total AC input current can end up being too low (under charging the battery). https://www.victronenergy.com/live/ve.bus:manual_parallel_and_three_phase_systems

I've played around with it in the past and could not get it to work 'dynamically'. You are going to have to get NodeRed up and running to do what you explained above.

Sunsynk uses high-frequency switching to create a AC sine wave. This design is prone to dipping since it takes time for the control-loop to catch up and tell those mosfets to giddy-up. Your battery is a tad small for this size installation. You are barely over the 1C battery/inverter ratio. 1.5C is the minimum in my opinion. Funny enough I have friend with the same problem on a 8kw Sunsynk. Also dips and is undersized (1.275C) on the battery.

My CerboGX just continues on as if nothing is wrong. I have another installation at my office with a VenusGX that also just continues on when the internet drops. So this is rather odd. Both above running Multiplus 2. My home one has two 5kw units running in parallel.

Grid use for 2022: This year to date: Winter 2023 was a rather wet and cloudy affair compared to previous years. Since I do not export to the grid, as soon as the batteries are full the mppt units pull back. So Solar totals can be much higher if allowed to export. From the start of 2023 I've gotten a lot 'smarter' in managing loads with automation and scheduling. Home Assistant pulls data directly form the Victron Cerbo and now switches the geysers, pool pump etc on and off. This is where the savings happen. Switching your big consumers to times with ample sunlight and free power. More panels are going up over the summertime to help with generation in winter. Currently got 5760W on the roof and they have been there since 2014. Going to add another 4kW.

Your meter has now been updated with two key change reset tokens. https://infrastructurenews.co.za/2023/01/25/24-months-to-go-if-not-reset-prepaid-meters-will-stop-working/

My father used to own a metering company with a few thousand meters. Yes you get flagged for no usage by the system and then get inspected. You also get flagged if your consumption drops more than 80% over a period of 6 months. There is no way to tell if the prepayment meter is consuming electricity other than tracking the amount and interval at which units are purchased. In short there is no communication with the prepayment meter. Its all tracked on the vending side. By buying small amounts at regular intervals is the best way to beat the system. This drags your long term average down without triggering the system. Just be aware that these prepayment meters will run down and consume units if you export to the grid. They only 'spin' one direction.