Fronius PV inverter on output vs bigger size Multiplus

I see some people using a PV inverter on the output/load side of a Multiplus. Is this to drive greater loads than what the Multiplus on its own might be able to?

In order to drive a greater load than what the MP is able to, what would be better?

  1. Put 2xMP in parallel
  2. Keep the 1xMP and add a PV inverter on the MP output/between MP and loads

What are the advantages/disadvantages of each?

If the backup power requirement is nor larger than you one MP, then adding a PV inverter would be more efficient and cost less. But if there are no PV power at night your max will be limited to the MP.
The PV will only contribute to what you need during the day when the sun shines.

It depends on how you weight the different aspects of a PV inverter.

A PV inverter on the load side should not be larger than the MP. ( the 1:1 rule)
There should also be around 4-5 kWh of battery per kW of PV inverter ( depending on battery chemistry).
Yes, you can drive a greater load than the MP can do alone, but obviously, only when the sun is shining. Two MPs can drive a greater load at night.
Using the power as you make it means the PV inverter is noticeably more efficient.
A PV inverter turns off at night, so there is no standing load. An MP requires a small amount of standing power whether you use it or not. (This was more of an issue with the older MPs).
PV inverters are a deal cheaper than a second MP and can be cheaper than most comparatively sized MPPTs.
PV inverters of 3kW+ have two inbuilt high-voltage MPPTs. The entire inverter will cost less than a Victron HV MPPT.
PV inverters are designed to work flat out without heating issues that cause downrating as often as MPs and MPPTs do.
Some PV inverters ( not all) can limit export power to the grid, and conversely, some can’t be prevented from exporting power, which can be a problem.

PV inverters are AC coupled at 230V, which usually means existing house wiring can be used.
Due to the higher voltage, wiring losses are less, so the PV inverter can be mounted fairly distant from the MP and batteries.
PV inverters can still charge the batteries as an MP is a bi-directional device.
A second MP usually means new DC cabling of substantial copper. Due to the lower dc voltages involved and load sharing from the same battery, a second MP has to be mounted close to and cabled equidistant from the battery as the first MP.

A second MP has to be of identical model and firmware to be paralleled. A PV inverter of any brand will synchronize to any MP.
An MP is a low-frequency device with more motor starting capability than a PV inverter (an HF device).
A PV inverter should use an expensive Type B earth leakage, whilst 2 MPs wouldn’t need this type of earth leakage.

There that is about all I can think of off the cuff.


Thanks for the detailed answer!

My DB is split into essentials and non-essentials.
If the non-essentials can all just be available in the day, then from what you say, it seems a PV inverter would be a simpler/less expensive solution.

Only caveat I see is that almost everything on my non-essential side have motors to start. Pool pump, heater pump, dishwasher, clothes washer and dryer…

But, I presume if extra starting power is needed, it will be contributed to by the Multiplus until the motor is running and then it will be mostly PV inverter thereafter?
I also presume if PV inverter power suddenly reduces (clouds), while say the pool pump is running, the slack will also be taken up by the MP?

I would also advise against actually doing that, or at least designing the system as if this will always work, because a PV-inverter can disconnect for some reason, and when that happens, if your loads exceed what the Multi can provide, it’s going to overload.

The reason for using a PV-inverter is (as you said) because it is more efficient when the power is used directly (most of your loads run during the day), and the reason to put it on the output is so you can still use it when there is a power failure, or because of the mentioned high voltage advantages.

Even in an off-grid setup, the direct-use and high voltage advantages still apply, but it is still advisable to size the Multi for the maximum load, for system stability.


I haven’t actually experienced that.

I can’t see you being in a less capable position with the PV inverter than without it.

In Oz, where having a grid-tied PV inverter is the norm, they also over panel the PV inverter by up to 33%. They would probably go higher but that ceiling is set by the contract with the utility.

What I do is over panel with an East array on one MPPT and a West array on the other.
For example, the ABB 3.6kW aurora allows up to 3000W on each MPPT.
Now, it will clip when the total output is 3.6kW, but it allows for a lot of tolerance for clouds.
It also allows me to get power earlier and keep getting it until later.

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So from my understanding, a PV inverter is something like “an unreliable” grid of a sort…with scheduled load shedding during night and un-scheduled load shedding during very cloudy and rainy days.
Beneficial thing is the low amount of loss in inversion, again during the day only…

I think a second MP would be more beneficial with adequate battery capacity.

Just my 2c :woozy_face:

I like to minimize complexity as far as possible…so it seems to me that a second MP, or larger MP, will be less complex…

Just another .5 of a cent :smiley:

When there are 2 MP’s you have redundancy in case one of them breaking :wink:

Both have their pros and cons, the situation and your usage pattern can dictate the more suitable option.
It is just another tool that can be deployed.

Remeber it’s only setup and configuration, a once off, that is a bit more complex. Running the PV combo is hands off and fully automated.



About half the time when load shedding kicks in or ends my Fronius Primo disconnects for a minute of two. Even if that didn’t happen, a cloud can move in front of the sun at any moment and then my MP2 is on its own.


I am not saying it doesn’t happen. I am saying it doesn’t happen to me.

But taking your own experience into account.
If you don’t have a PV inverter and you overload the Multi, it’s guaranteed to switch off.
With a PV inverter, it maybe switches off. I don’t follow the logic that it is better to sit definitely without power instead of maybe sitting without power.

Clouds happen, but so does sunshine.
So when you get your load-shedding, and the fussy Fronius reconnects on a sunny day, are you better or worse off?
(That sunny day during load-shedding when you really need the aircon).
A DC MPPT doesn’t work in the shade either, but also can’t give the inverter a dig-out.

I don’t think you should definitely be in one camp or another.
What about that lovely outbuilding roof that is 60m+ away from the batteries? It will cost a comparative fortune to DC couple any PV you install on that roof.
Whereas a PV inverter you install in the outbuilding and supply the existing AC wiring.


Having a PV inverter is better than not having one, no argument there. The OP’s question is whether a PV inverter is better than another MP2 when the aim is to carry larger loads, and my answer to that question would be an emphatic “no”.

I installed an MP2 + PV inverter setup when loadshedding was still an occasional nuisance. Now that I have to deal with being off-grid for almost half the day I feel another MP2 would have been a better choice. It’s really tiring having to manage loads all the time.

Honestly, if I knew back then what I know now I probably would have gotten both a second MP2 and the PV inverter.


“better” is a very subjective term.
Without budget limits and with the correct topography, you are right, but who has that?
Every situation has to weigh on its own merits. Your situation is not the same as the OP’s.
Being aware of available options can never hurt.

People have budgets. A second inverter requires basically doubling your initial investment because you have to get the same again to parallel.
If I have a 5kVA inverter or a 15kVA inverter and I am 2 kVA short sometimes becomes quite an investment.
That distant roof with no shade is a real thing in many cases.
So as I have said, it is another tool, especially for those who wish they just had a bit more power sometimes.
As I said:

But it isn’t a universal fix-all.
To add some perspective, if you line up all the “ifs”. If my inverter switches off, if there are clouds if my loads are too high at that exact time.
The perfect storm of “ifs” will still give you a more reliable supply than the national utility with a PV inverter.

So my take on the OPs question:

If you in fact need more inverting capacity as a rule, you get another battery based inverter… and more batteries. This is more expensive.

If you are happy with what you’ve got on backup, but want some more panels (and want less strain on the Multi while the grid is down and it is sunny), get the PV inverter for the new panels.

If you are going to get a new battery based inverter, batteries, an MPPT and solar panels, then you are spending a lot of money.

Should I be getting more panels, I’ll definitely be adding them on a PV inverter. My Multi is good enough for my guaranteed backup.

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I agree with this. My point is merely not to DESIGN this assumption into the system. If you occasionally or even frequently get away with it (and you will), great! But if you get the occasional trip, don’t come crying because “so and so on the interwebz said this should work” :joy:

While back I asked about panels and a Solis, this was EXACTLY my thought too.

As a matter of fact, if we extend the house, 2.2 people in the new addition, the new people need to buy a Solis and some panels, tied into the existing Victron system. Sorts the additional load nicely, on sunny days.

May even give them access to some batteries too in the evenings with LS.

But only a little bit, ok.

@plonkster, I want to continue exploring the counterintuitive nature of the regulations about an unapproved Victron with an approved PV inverter downstream.

So I brought the question to this thread so the “SMA for sale” thread wouldn’t get cluttered.
Perhaps other comments in that thread need moving as well.

Firstly, do you have a written reference to this interpretation of the regulations?
Not that I doubt that this reasoning has been used in the past.
As it is so counterintuitive, if a person took advantage of it, they’d be asked to prove it by anybody with some technical sense. I want to make sure it isn’t some non-technical government official’s personal interpretation.

Secondly, if this is the case, it becomes the best argument ever for AC coupling. Because with an approved PV inverter, you’d have carte blanche to use Victron’s entire range of hybrid inverters.(Approved or not)
Surely, there must be a catch?