Will an AC Coupled Fronius Primo be worth it for my use case


  • Over the last 2 years I’ve reduced consumption as much as possible and moved as much load as possible to the daytime in preparation for PV being added.
  • I have a 5kVA MultiPlus-II with 10.5kWh of Pylontech batteries and will be adding PV within the next 3 months.
  • I’m using 545W 72-cell panels for my example.
  • Due to shading and roof space I can have 2x panels on a North facing roof and the rest on a West facing roof. (Potentially 2x on a West facing roof which only gets sun access the same time as the North facing which I’m still pondering on.)
  • From my consumption (14 - 18 kWh total per day) I should be OK with ~4.3 - 5.4kWp of PV to manage the loads and charge back up.
  • ~1kWp will be on a Victron MPPT on the North facing roof. (with a second string that can be paralleled if needed either West or East)
  • ~3.2 - 3.8kWp will be on the West facing roof.


  • Below is my daytime consumption. This was during a laundry day hence the frequent peaks, but is generally the day with the most daytime consumption. The only caveats here are that the swimming pool was switched off between 2pm and 4pm when it generally still runs (in the summer) and that I took a late shower hence the load after 4pm:

  • During the winter months the pool runs for shorter periods and can be moved to accommodate the West facing array’s production times

  • We will most likely be adding a tumble dryer

Will a 3.0 - 3.6kW Fronius Primo make sense for me with such loads or should I just put the rest of the panels on a second Victron MPPT?

I’ve always liked the idea of having AC coupled PV with most loads during the day and using spare and the single MPPT to charge the batteries, but I’m not sure if it will make sense with my loads being quite low. (Should I switch back from heat pump to heating element? :wink: )


  • The 3.0 - 3.6kW Fronius Primo shows Usable MPP voltage range of 80 - 800 V, but MPP voltage range of 200 - 800 V. Like @plonkster previously mentioned in Fronius MPP Voltage Range you need to aim for a higher voltage.

  • With 6x panels in series the voltages etc. looks as follows:

  • And with 7x panels:

  • I think it’s worth throwing the efficiency curve in here too. Yes, it’s for a larger, slight more efficient model, but I think the idea remains the same accross model where higher Vdc means higher efficiency

  • I don’t necessarily want to undersize the PV inverter. I’d rather leave room to add more, so I’m leaning towards a 3.6kW Fronius Primo instead of a 3.0 / 3.5kW.

I think if you use the majarity of your power in the day time and you already have the MP II for the battery backup, then adding extra PV using a PV inverter makes a lot of sense. In essence you are not only increasing your PV, but also your inverter size.

There are 2 scenarios where it does not make sense.

  1. if your night time loads are too high for your MP II, but that does not look like the case here.
  2. if the sun does not shine and you still want to use more battery/inverter power during the day.

That efficiency curve on the Fronius is also extremely hard to argue with. If you have daytime loads that this can service… go for it!

I have a Primo 5.0 which until recently had four 72-cell panels in series (±150V operating votage) on one MPPT and eight panels (±300V) on the other. The peak output on the four panel string was almost exactly half that of the eight panel string. I don’t know how it would perform with 3 or less panels, but it did just fine with 4.

The price difference between the 3.6 and the 5.0 is only slightly more than the cost of a single solar panel, so I would advise you to consider that. Not only for future proofing your system, but I would also expect it to last longer if it is running significantly under capacity.

The Primo really is an awesome piece of kit. The only thing that annoys me is that it sometimes disconnects for a minute or two when loadshedding hits and it almost always does that when loadshedding ends.


Isn’t that part of the grid code? It wouldn’t be allowed to generator at maximum capacity for 10 minutes actually after LS ends.

A Fronius Primo on the output of a MP2 is set to grid code MG50 (Microgrid 50Hz), so those rules don’t apply to it.

My current theory is that it picks up a glitch in the AC waveform when the MP2 reconnects to the Eskom grid and then the Primo promptly disconnects itself. I can hear the clicking of the Primo’s relays opening when it happens. When it reconnects a minute or two later it ramps up to 100% output almost immediately.

Yup, that’s exactly what it is. When the grid returns, the Multi has to sync with the grid frequency. If you listen carefully you can even hear it. It does this by speeding up the frequency slightly for a few cycles until the two 50Hz signals are in phase (so that closing the transfer switch becomes seamless).

But this frequency syncing is detected as a vector shift by the Fronius and it does what it must, it anti-islands (that’s a weird verb, I admit).


Is there any chance the sync process can be slowed down (or otherwise improved) so that it flies under the Fronius’ radar and it doesn’t disconnect? The MP2 already takes a while to reconnect, so a few seconds more would really not matter that much.

I’m sure it is, but you can be sure that will be dead last on the list of priorities! :slight_smile:

Thank you so much for all the responses folks!

I did not even think of that.
That’s actually a nice plus although not something that will be used all that often. When the inverter & battery size was selected that was built around a stable-ish grid with the occasional loadshedding. As loadshedding has become part of our daily lives I do get the occasional overload warning when it’s midday and I have the pool running, heat pump running and the lawnmower runs at the same time the microwave is started. Not that frequently and only a minute before being back to normal, but nice to know that if the sun shines it will help with that seeing how the Multi will then only need to provide the shortfall.

That’s sort of where I’m a little stuck. I have most of my loads during the day, yes. But it’s only a ~1kW sustained load with it going to ~2kW for 30 minutes to an hour when the heat pump switches on. There are 3kW spikes on laundry days and a tumble dryer on the horizon though. The rest of the PV generation will go via the Multi in to the batteries. But I guess it’s still worth it?

That is not a bad idea and something I was considering, but I’m a little worried about battery capacity. I’ll add a separate comment about that after this “reply to all” so that it doesn’t get missed.

What firmware versions are you running on the Multi & Primo?
Not 100% sure, but there’s always a chance a firmware update made some changes to mayyybe change things.

So while I have everyone here and there is a general consensus that the Primo is not a bad idea (I actually thought it’ll go the other way and “just slap some MPPTs on” was going to come out.

Up next is discussing the minimum battery capacity section of AC-coupling and the Factor 1.0 rule [Victron Energy].

To quote:

3. Minimum battery capacity

Besides the relation between installed PV Power and the inverter/charger VA rating, it is also important to have a sufficiently sized battery. The minimum battery capacity depends on the type of battery, lead or lithium.

Note that, besides the minimum battery capacity, the mentioned sizes are often also the most economical battery size. In case used for self-consumption purposes that is. In case the goal is to increase autonomy, of course installing a large battery increases the system autonomy in case of a grid failure.

3.1 Lead batteries


3.2 Lithium batteries

1,5 kWp installed AC PV power requires 4.8 kWh of battery storage:

  • 100 Ah at 48 Vdc

  • 200 Ah at 24 Vdc

  • 400 Ah at 12 Vdc

Each additional 1.5 kWp of AC PV will require an additional proportional 4.8 kWh increase in battery storage.

In my case I have 3x US3000C Pylontech batteries with 10 656 Wh nominal capacity. Following the above that means I’m limited to 3 330Wp of PV. Is that correct and something to generally stick to? I guess it’s a case of is it best to stick to that general recommendation or is there something more specific that can be worked on knowing it’s a 5kVa Multi-II with 3x US300C Pylons?
Not to sound like our former president, but they “made recommendations and recommendations are recommendations, they are not verdicts. Recommendations are recommendations subject to be taken or not taken if they are recommendations.”

Just to add:
I don’t know enough to work it out for myself and I’m OK with keeping it at that number. But if someone with more knowledge can shed some light on it that will be appreciated.
What I can add is that with the 3x Pylons, when the batteries are near full, the BMS has a Charge Voltage Limit (CVL) of 53.2V and a Charge Current Limit (CCL) of 44.4A which translates to 2 362W. Victron’s 52.4V cap translates to 2 326W.
The Pylons have a recommended charge current of 111A which is 5 328W at their 48V nominal voltage and 5 816W at 52.4V.
The Multi-II has a max charge current of 70A which is 3 360W at 48V and 3 668W at 52.4V.

I am generally OK with that. I can install a Primo 3.6-1 and throw 9x 385W 60-cell panels on it for 3 470Wp (which should be OK on a West roof facing slightly South) and add 2x of them North on an MPPT to get to 4.3kWp and will still have above grid voltage MPP range on the Primo. Or even a Primo 5.0-1 like @PierreJ suggested and then add an extra battery and panels later if needed.

Yes. Has to do with the fact that the inverter is of an LF design. High power levels cause DC ripple. Also, in an off-grid scenario the Multi needs some time to ramp the frequency to control the power level of the PV-inverter, and there is a filter on this, it cannot move too fast (otherwise the PV-inverter’s RoCoF detection will cause it to switch off), so your battery needs a bit of capacity to deal with additional energy that might flow into it while the frequency is shifting.

So if you bend the rules a little you will probably be fine, especially with some loads on the output too, but don’t go too far with that :slight_smile:


The latest on both.

I had 4.8kWp panels on my Primo 5.0 connected on the output side of my MP2 5kVA, together with 5 x Pylontech US2000 batteries (12kWh). That’s 3.75kWh/1.5kWp. I didn’t experience any issues with DC ripple or high battery voltages.

I reckon there’s a fair amount of safety margin built into the recommendation of 4.8kWh/1.5kWp. Venus OS limits the charge voltage of Pylontech batteries to 52.4V, which adds an additional buffer.


This is important, my battery tripped during my already short holiday:

  1. Battery was basically full for a couple of days
  2. It was during a load-shedding window
  3. Pool pump was running, everything nicely balanced
  4. Pool pump turns off and
  5. Battery voltage spikes, that power needs to go somewhere while the PV throttles down
  6. :boom: BMS trips battery breaker due to overvoltage
  7. Drive back 400kms

I’ve since slapped a “Holiday Mode” switch together with Venus Large’s Node-Red to stop charging at 98%, ensuring that headroom is there. Holidays are never so long that 98% will be an issue. :cry:

(The drive back was only necessary because the main breaker ALSO tripped, along with an EL that goes to the borehole and booster pump. Alarm was last to go with bip… bip… bip… squeeeeeeee. Silence.)

Ah, I see I started up my reply, but never finished it. The amount of times I get distracted with something and forget to finish what I was busy with (don’t tell my boss).

I guess that’s where I’m a little confused. My battery will definitely be at 100% at 4pm which will be when the sun is still shining nicely on the West panels that will be connected to the Fronius to supply my ~1200W load. That’s also when the pool pump will switch off, dropping the load to ~400W.
Or another example where I had 3700W of load at midday (most likely also 100% battery by then) and a drop to 1850W.

I’m assuming as it’s not as big a shift it’ll be OK and the batteries will be able to handle it without an issue? (I’m planning to barely go over the Wp limit)

That’s exactly the concern that I’ve got.

This has only happened to me once in about 6 months of running the system.

I’m also 50% over-panelled, so 7500Wp with the Primo 5.0.

And as I said, the battery was basically full with very very low loads for a couple of days. This has never happened while cycling the battery daily, even when all the other things are the same.

So I wouldn’t worry for 99% of the time. Just need a little something for that special case, which is easy with Node-Red, everything’s built in.

flows.json.zip (3.0 KB)

I always leave a key with someone. You can do a lot with a WhatsApp video call.

I blame VRM. Makes you windgat.

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