Backup power with an existing MultiPlus-II: a pragmatic solution for a heat pump and well?

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1

Hello everyone,

A conversation with my wife recently got me thinking.

Following the acts of sabotage against infrastructure here in the Rhenish lignite-mining region, she remarked how reassuring it was that, with our PV system and battery storage, we would actually be well prepared for a prolonged power cut.

I then had to explain to her that, although our system has almost 25 kWp of PV capacity and more than 22 kWh of battery storage in total, it would nevertheless largely fail in an emergency. All of the PV inverters are SMA units and are grid-following. If the public grid fails, the entire PV generation therefore also shuts down. Unfortunately, the same applies to the existing SMA storage solution.

I am aware of this drawback. A complete solution with a three-phase islanded grid and continued PV operation would be a much larger project. More on that later. For now, I am concerned with a much smaller and more pragmatic solution.

Objective

For the time being, I do not want to make the entire house emergency-power capable straight away. I would nevertheless like to explore that conceptually as well; see the follow-up posts.

For an initial stage, the following loads would be sufficient for me:

  • air-to-water heat pump
  • groundwater well for the drinking-water supply
  • chest freezer
  • some lighting
  • router, mobile phones and similar small devices

Existing Victron equipment

In addition to my SMA system, I already operate:

  • Victron MultiPlus-II 48/5000/70-50
  • Cerbo GX
  • 15 kWh Felicity battery storage system

The MultiPlus currently operates exclusively as an AC-coupled storage system. The AC-Out has not been used so far.
(Details of the SMA/Victron setup are in the first reply to this post for anyone interested…)

My idea

I would like to use the AC-Out as a separate emergency-power network.

The following would be connected to it:

  • a CEE 32 A socket for the heat pump
  • several Schuko sockets for the chest freezer, lighting, etc.
  • the groundwater pump
  • if required, a cable reel for other important loads in the house

Generator

I would like to install a changeover switch on the MultiPlus’s AC-In:

Grid ↔ Generator

At present, I am planning to use a Könner & Söhnen KS 5500iES ATSR with remote-start functionality as the generator.

The generator would not supply any loads directly, but would exclusively supply the MultiPlus.

This would mean that all loads would always see the supply generated by the Victron. Depending on the situation, the energy would then come from the battery, the generator or a combination of the two.

The generator should only start automatically if the battery level becomes too low or the required power exceeds the MultiPlus’s continuous output.

Heat pump

The installed unit is a Nibe F2125-12.

The heat pump can easily be operated on a single phase. The phase used for this is protected by a 25 A circuit breaker, while the heat pump requires a maximum of around 20 A. The electric auxiliary heater would of course not be supplied in emergency-power mode.

I would install a changeover switch at the heat pump:

Grid supply ↔ Emergency-power supply

In an emergency, the heat pump would then be supplied via a CEE connection from the MultiPlus’s AC-Out.

The plan is to use a CEE extension cable approximately 15 m long with a cross-sectional area of 4 mm². It would only be run from the basement window to the garage when needed and would not be installed permanently.

The actual question

Am I overlooking any fundamental flaw in this concept?

I am particularly interested in experience with:

  • using a MultiPlus-II as an emergency-power supply for a heat pump
  • generator operation via the AC-In
  • automatic generator control
  • possible pitfalls or protection concepts
  • anything that would immediately stand out to an experienced Victron user with this setup

I am explicitly looking for a robust and as-simple-as-possible solution for a prolonged grid outage, not yet a complete off-grid system for the entire house. I will discuss those considerations in the next posts.

3

As the question will probably come up as to why I operate an SMA system and a Victron system in parallel at all, here is some background.

My system was originally built entirely with SMA equipment in 2019:

  • Sunny Tripower 9000
  • approx. 9.8 kWp rooftop system
  • Sunny Boy Storage 5.0
  • BYD high-voltage battery with 7.7 kWh
  • SMA Home Manager 2.0

When I later wanted to expand the storage system, the official SMA route seemed economically unattractive to me. In the end, I was advised to largely rebuild the storage system and inverter setup. That was far too expensive for me.

So I decided to take a different approach.

I sourced a used STP 5000 and an STP 8000 from the same generation of equipment. Both came from system owners who had switched to hybrid inverters. I installed the units myself and then had them inspected and approved by an electrician.

They are now connected to:

  • 6.5 kWp south-facing array on the workshop shed in my garden (partially shaded in the morning and evening)
  • 8.8 kWp west-facing array with a 15° pitch on the garage

Together with the original rooftop system, this gives me just under 25 kWp of installed PV capacity.

Storage expansion with Victron

For the storage expansion, I opted for Victron:

  • Victron MultiPlus-II 48/5000/70-50
  • Cerbo GX
  • Felicity 15 kWh battery

The Victron is in the garage, directly next to the STP 8000. Both units are connected to the house distribution board via a shared cable.

The MultiPlus is currently operated exclusively as an AC-coupled storage system. The AC output is not being used yet.

Operating SMA and Victron in parallel

Since SMA ESS and Victron ESS do not really understand each other, I built my own solution for this.

A Node-RED flow runs on the Cerbo and distributes the charging and discharging power between the two storage systems.

Specifically:

  • during charging, power is distributed according to available storage capacity
  • during discharging, power is distributed according to the energy content available in each system

The Cerbo can calculate the required data because it sees all three PV inverters, knows the grid connection point and can also detect how much the Sunny Boy Storage is currently charging or discharging.

The SMA system effectively does not notice anything from the Victron system. From SMA’s perspective, only the household consumption changes.

Conversely, Victron knows about:

  • the MultiPlus
  • the Felicity battery
  • the SMA inverters

However, Victron incorrectly recognises the Sunny Boy Storage as another inverter or PV generator, rather than as a second battery system. As a result, Victron ESS does not work in my setup without custom logic.

Nevertheless, it has now been running extremely reliably and significantly better than many people initially predicted to me.

Visualisation and energy management

Neither manufacturer’s system displays the entire installation correctly.

The SMA app only knows about the SMA components and knows nothing about the Victron storage system.

I therefore only have a complete overview of the entire installation in:

  • Home Assistant
  • OpenWB

There, all three PV systems, both battery systems, household consumption, grid import and export, and vehicle charging are shown together.

OpenWB in particular does this surprisingly well and is my central energy management system for surplus charging.

Other loads

The following are currently charged via it:

  • VW ID.7
  • VW ID.3

using an OpenWB 22 kW charger.

There is also a Nibe F2125-12 air-to-water heat pump in the house.

Conclusion

Overall, I am very happy with the current setup.

With a manageable financial outlay, I was able to significantly expand the existing SMA system without having to replace the equipment already in place. The combination of SMA, Victron, Node-RED, Home Assistant and OpenWB works reliably.

The only genuinely major disadvantage, however, is that practically the entire system would shut down in the event of a real grid outage.

Although the generator solution described in the main post would give me a robust backup power supply for the most important loads, I would still be unable to use the existing PV capacity of just under 25 kWp.

That is precisely why I am also considering a much larger off-grid solution. More on that in the next post.

5

You would also have to reprogram the MultiPlus every time so that it accepts the generator.
With the German grid code, which is required for operation as an ESS, it will almost certainly not connect the generator.

Unfortunately, that is a common misconception.
Whatever is present at AC IN is passed through 1:1 to AC OUT; you can actually see this quite clearly in the MultiPlus block diagram.

The control system is too slow for that.
If you get a load spike and call for the generator, it takes a good 1–2 minutes for the generator to start, for its voltage to stabilise, for the Multi to synchronise and for the connection to be switched through.
By then, the load spike will already have passed, or the Multi will have shut down due to overload.

The system should be designed so that the Multi can handle all the load spikes that occur by itself.

Is that just the heat pump, or does it include the heating system?
If that figure applies to the heat pump alone, that is already too much for the MultiPlus.
You should measure this beforehand.
Depending on how the heat pump starts up, its starting current could also trip the MultiPlus, especially if other loads are running at the same time.

Without knowing more about the local conditions, I would almost say that it would make more sense to convert the Victron system to three-phase operation and connect the SMA system to its output.
That way, you could continue using the “old” SMA system, including the PV system, in the event of a grid failure.
Only the two additional SMA units would probably not be allowed to run on AC OUT because of the 1.0 factor rule.

That would also save you the generator, which would need regular maintenance and test runs to ensure that it actually works when needed.

For the price of the generator, you could probably also buy the two additional MultiPlus units.

If the heat pump can be connected to three-phase power, this setup should also allow you to run it without any problems during a grid failure.

6

I wouldn’t fully integrate the heat pump, otherwise the batteries will be flat after about five hours in freezing conditions. I’d only run the circulation pumps so that the system doesn’t freeze.

7

What’s connected to ACout 2?

Do you have the option of switching something on there (the second AC-PV system) and thereby reducing the power output if the grid fails (in the event of a grid failure and the resulting island operation, and depending on the battery charge level, possibly switching it back on via Node-RED)?.

8

Hello Matthias,

Many thanks for the detailed reply, which unfortunately has led to even more questions :grinning_face_with_smiling_eyes:.

The background is that I’m still not entirely sure whether I’ll stick with my current “garage solution” or move all the Victron equipment into the cellar. There, I could imagine expanding to a 3-phase system with two additional MP2s and another battery of around 15 kWh.

This raises a few questions for me:

1. 3-phase expansion in the cellar

If I expand to 3 MP2s and run the entire house through the Victrons:

  • Would I then need an external changeover switch between grid-connected and off-grid operation at all?
  • If so, why exactly?
  • What does the changeover switch do that the MP2s cannot already do themselves?

This isn’t entirely clear to me yet.

2. MultiPlus size and mixed operation

You suggested that the heat pump could become critical.

  • If I were building the system from scratch, would you recommend going straight for MP2 8000s?
  • Is it permissible to use devices of different sizes in a 3-phase setup, e.g. a larger MP2 on the phase serving the heat pump and smaller ones on the other phases?

Or do they have to be identical in a 3-phase system?

3. Heat pump

Unfortunately, the heat pump (NIBE S2125) is single-phase only.

The good news is that I can limit its maximum power consumption.

The approximately 20 A I mentioned refers to the heat pump itself, not to an immersion heater. The immersion heater could be switched off completely in emergency operation.

4. Separate MP2 just for the heat pump?

A slightly crazy idea:

Could the heat pump, if necessary, be connected to a fourth MP2 that is not part of the 3-phase backup system at all, but only supplies the heat pump?

Or would that conflict with the overall architecture of an ESS?

5. I apparently haven’t fully understood AC-In / AC-Out

Your explanation got me thinking.

If I understand it correctly:

  • In the event of a grid failure, the MP2 disconnects AC-In and creates its own grid on AC-Out, with its own frequency and voltage regulation.
  • However, as long as a valid grid is present at AC-In, it is simply passed through and the MP2 operates synchronously with it.

Is that correct?

Put another way:

If an inverter generator is connected to AC-In instead of the DNO grid, then the generator determines the frequency and voltage, and the MP2 can ultimately only feed additional power into this grid, but can no longer set the grid parameters itself?

If so:

How certain is it that an MP2 in German ESS/Grid-Code mode will actually reject an inverter generator?

Are there any practical experiences with this?

And could the same issue also apply to the heat pump, which is presumably rather sensitive to power quality as well?

6. Alternative: generator only for charging the battery

That’s why I had the idea of using a Skylla or another dedicated 48 V charger instead of connecting the generator to the MP2’s AC-In.

The generator could then charge the battery exclusively, while the MP2 would remain “master of its own grid”.

Would that be more robust in your view?

In that case, one could presumably also use a simpler sound-insulated diesel generator with an ATS, without grid-code requirements or synchronisation playing a role.

7. SMA PV in off-grid operation

One more question to make sure I understand:

Suppose I do eventually switch to 3-phase off-grid operation.

In off-grid operation, there would then be nothing connected to AC-In, either because the MP2s have disconnected or because a changeover switch has already disconnected the grid.

If the existing SMA inverters were then connected behind the Victrons on AC-Out:

Could surplus PV power still be used to charge the 48 V battery via the Victrons?

I’m currently trying to understand precisely how SMA PV, Victron and the battery interact in off-grid operation.

Thanks again for your help. I realise that I’m currently wavering somewhat between a “pragmatic backup-power solution” and a “fully fledged off-grid system for a detached house”, so the questions are still evolving a little. :grinning_face_with_smiling_eyes:
Best regards,
Hannes

9

You need the transfer switch for maintenance purposes. If one of the three MultiPluses fails, all three switch off and you’ll be left in the dark. The same applies if you ever need to work on the system, even if it’s only for software updates.

The MP2s have to be identical.

Yes. The Victrons can route PV power from AC_Out to the batteries. The factor 1 rule must be observed here, as Matthias mentioned.
As long as the SMA inverters are connected to AC_In, they are dead as soon as the MP2s switch to off-grid operation. You already have that problem now: when the power goes out, nothing works. On AC_Out1, they can still feed into the off-grid network created by the MP2 even in the event of a power cut.

I would proceed as Matthias suggested. I would replace the two additional SMA inverters with MPPTs (provided this is possible in terms of voltage). The entire system would then be capable of a black start even when the batteries are empty.

10

I would first test that with the existing MP and see whether and how well it works.
As already mentioned, the critical issue is the starting current, especially if other loads are running at the same time.

That could be an option.
Using a changeover switch, you could switch the HP to the Multi only when needed, while the Multi would otherwise always be switched off.
You could perhaps also use this Multi for charging from the generator.
If you can do without the HP for a while during generator charging, you could certainly arrange it so that the HP is disconnected whenever the generator is running.

Könner & Söhnen also has a DC generator in its range.

However, if you go three-phase, I would put the generator issue on the back burner for now.
You would then essentially have 9.8 kWp and 22 kWh from the “old” system, plus 30 kWh from the Victron system (with the 15 kWh expansion you mentioned).
That should already be quite workable if, during a grid outage, you try to minimise energy consumption to the essentials.

As Esilar wrote, you could also reconfigure the PV capacity of the STP 5000 and/or the STP 8000 to one or more MPPT RS 450/100 or 200 units, allowing you to use even more PV capacity during a grid outage.
However, that depends very heavily on how the PV strings are currently divided and whether they can be reconnected without too much effort.

This could potentially eliminate the need for a generator altogether.

11

You can also connect a charger or telecom PSU to any generator to charge the battery. Huawei or Eltek Flatpack2 are typical 24/7 PSUs and can be bought on eBay for around €100. They’re noisy, but should be sufficient in an emergency.

Some people say that basic generators can’t withstand continuous operation. Industrial diesel generators are much more expensive.

I went for a similar setup—just AC-PV, for 3x MP2 5000—but I don’t have a heat pump. The MP2 6k5 or 8000 might be better suited in that case.

12

Hi all,

the bit about the MPTTs isn’t entirely clear to me. As I said, I currently have an SMA system with three inverters, all of which I want to keep and continue running in the event of a power cut. (Limited to a total output of 15 kW via HM2.)
And so far, I’ve only been using the MP2 as an ESS; all it does is charge and discharge the Felicity.
Now I’m thinking of adding a bit more Victron equipment, but I don’t want to switch over completely.

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This would be my idea: a backup grid made up of three MP2 5000s, so that the SMA equipment remains operational, including the SBS50 with the 7.7 kWh BYD battery connected to it. The latter would then feed into the phase with the heat pump. Normally, it’s controlled by the HM2 and tries to supply the consumption across all three phases and achieve a net balance of 0 at the grid connection point, while creating a considerable phase imbalance. This can be limited; I’d set it to, say, 200 W, so it would effectively only work for L1. Does that make sense, or have I overlooked something? And if this doesn’t work because the SBS5.0 doesn’t like the Victron grid and is disabled in backup-power mode, I might need a dedicated fourth MP2 for the heat pump. Any thoughts?
BTW, thanks in advance for all the comments so far!

13

The problem is the 1.0 factor rule mentioned several times already.

Accordingly, you must not operate your entire SMA system on AC OUT.

14

So, if I understand this correctly, I’m only allowed to feed in 4,000 W per MP2. That would be 12,000 W in total. If I limited the system to that in the off-grid scenario, would that work?
I found this:

Cerbo GX (Venus OS): The Victron system immediately detects when the grid goes down. Via the VE.Bus status, the MultiPlus-II’s state changes from “Grid-connected” to “Inverter” (island mode). In newer Venus OS versions (from v3.50/v3.60 onwards), Victron has integrated a Dynamic PV Power Limiting function via SunSpec Modbus TCP. If the GX device detects island mode, it can directly throttle the SMA inverters via Modbus to protect the battery.

I would then set the 9000 to a maximum of 6 kW, the 8000 to a maximum of 4 kW, and the 5000 to a maximum of 2 kW.

15

Your plan is very complex with 7 inverters (?) this could lead to uncontrollable instability in the system, keep it more simple, use a more Victron only system; skip the SMA battery and generator, I would even substitute the sma inverters with Victron inverters.

Check true power consumption of the heatpump, often 20A is only the very short peakpower.

16

Max. 4 kW per phase. That’s the upper limit, and it shouldn’t be fully exploited. No software regulation or anything similar.
A defect caused by non-compliance with the 1.0 factor rule is NOT covered by the warranty!

17

I would connect the SMA9000 to AC_out and the SMA8000 to AC_in.
Or replace one of the SMAs with a Victron RS450.

If only 9000 W three-phase is connected to AC_out, then the aforementioned factor-1 rule is met. The battery would also be sufficient to provide buffering if the grid fails while the sun is shining and the SOC is at 100%.

The second SMA8000 on AC_in will operate normally as long as the grid is available. If the grid fails, it will switch off.

If you replace the SMA on AC_in with a Victron RS450, you will also be capable of a solar black start. However, you can achieve this with your planned DC generator as well.

Black start means starting up the system when the batteries are empty. The MP2s need a battery in order to synchronise with a grid, which is required by the AC PV (SMA). Victron RS450s or Victron MPPTs start based on PV input, so they can also start with an empty battery.

For this purpose, and to charge the battery more efficiently, I have an MPPT 150/45 with four panels.

I think I would sell the SBS and the 7 kWh battery and try to get a third one for the Victron.

18

hello,

a heat pump is basically a deal-breaker. Even a 5 kW heat pump needs around 2.5 kW at sufficiently low temperatures, so any battery will soon be flat! And yours apparently also has a very high starting current, which sounds like an older model! Inverter models normally don’t have such a high starting current!

I use air-conditioning units for heating, which gives me considerably more flexibility, and they also have a higher efficiency than an air-to-water heat pump, as they don’t need such high intermediate-circuit temperatures. At least, no one has been able to prove otherwise to me so far. Although a heating engineer promised me that there is a new heat pump that can achieve a flow temperature of 70°C at a COP of 5 when it’s cold outside. I haven’t got round to checking the data sheets yet, though. You hear all sorts of stories when people have plenty of time to talk.

I currently have a battery capacity of at least around 150 kWh, and depending on the load, it only shows a runtime of around 25–30 hours after sunset! I’ll probably also have to switch off the air-conditioning unit running in the cellar during the summer soon, as it draws around 1 kW continuously while it’s operating!

You’d be better off getting a way to heat with gas. However, not all appliances are suitable for use in a flat, and fresh air obviously needs to be able to get in. I certainly wouldn’t do this in an airtight flat, but we have plenty of air vents here—and of course, only use it under supervision! So it has to be an appliance that is permitted for use in a tent or living space as well.

If you want to use a generator, there are inverter models, but be careful with the ESS configuration. You might be better off setting the ESS to external and not controlling it, so it’s effectively switched off! Not every inverter survives when the Multi tries to feed power into the system.

It might be better to connect the generator to a separate charger; that could also be an additional Multi.

cheers

19

@ DF: Ooh, there’s certainly a lot of speculation going on here… It’s an inverter heat pump, the latest Nibe 2125 model (as mentioned further up), so it definitely doesn’t have a high starting current—that’s not the issue. It can draw up to 20 A, unfortunately on only one phase, according to the specifications. It should only need to keep running if someone sabotages the power grid here (hence my wife’s request for a generator). And the question was, and still is: will it work with an MP2 5000?

I didn’t ask whether I consider the heat pump economically viable (I do, with a flow temperature of max. 45°C for the radiator circuit and max. 30°C for the underfloor-heating circuit). And no, I don’t want to switch back to gas; and yes, I also have air-conditioning units in some rooms, which I occasionally use for heating, but I find them considerably less cosy than underfloor heating because of the airflow.

Best regards,
Hannes

20

Hello,

if this is a modern heat pump, then the maximum current of 20 A is surely with the immersion heater, or does the heat pump have around 12 kW of heating output? There are, of course, also 8/9 kW heat pumps that can draw that much current, but they still deliver their rated output at −10 or −15°C!

The MP2 can only supply around 18 A continuously. It’s best to stick to a continuous output of <= 4 kW. In any case, that’s how I’ve set up my systems!

But the other problem is the battery: with a continuous load of 1–3 kW, you already need at least a 60 kWh battery, or you have to start the generator in good time.

Bye