question

sesshoumaru avatar image
sesshoumaru asked

Building a backup power system

Hello

I have a 7.8 kWp solar system with a Fronius Symo 6-0-3-M inverter and a Varta Element 6 battery.

I want to add backup power capability to the system for cases of power grid failures.

The inverter is 3-phase coupled to the house and I want to provide backup power to the entire house on all three phases, within the limits of 16A per phase (230V). In addition there will be a diesel power generator (8KVA) as well, which shall be integrated. As battery I would consider a BYB LVS 12.0 kWh system for start.


Even after reading lots of information about Victron and possible configurations I still have a ton of questions. I hope somebody here is kind enough to help me through the thought process.


  1. If I want to supply all three phases with backup power, I have to have 3 Victron units?
  2. Can I use 1 Quattro for the generator to come into play on one phase (L1 e.g.) and for the other two phases use a multiplus?
  3. Will all three Victron units be conencted to the same battery stack via some master charge bus?
  4. If I keep the Fronius it would be AC coupled and deliver up to 6000W PV power, how does that affect the dimensionig of the Victrons? 3x3000 = 9000 (okay?) or each all units have to be capable of handling the full 6000W?
  5. I have an additional 5000W UPS for the most critical loads, so for this phase I would disable the Victron embedded UPS?


Thanks in advance :)

system design
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

5 Answers
marekp avatar image
marekp answered ·

ad 1. For 3 phase system you will need 3 identical units.

ad 2. No, you need 3 identical units.

ad 3. Yes

ad 4. 3 phase Victron system (3 x 3000) is big enough for AC coupled 6kW 3 phase Fronius.

If you need 16A x 230V = 3680W in each phase, than Victron 3x3000VA will not be big enough.

ad 5. This additional 5kW UPS can be connected after Victron system on the critical phase.

I have similar system and I am using diesel generator only for charging battery direct with the separate charger. This way the use of the generator is the most economical.


2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

sesshoumaru avatar image
sesshoumaru answered ·

@MarekP

Thanks for that very useful information. A pity that you can't have different units.

I thought the VA3000 meant that it only can take in 3000VA from a PV but still can put out more to the loads from the grid. So its more like a total capacity. Thats a very good hint.


You can load the battery (BYD LVS 12) directly from a diesel? Can you elaborate a bit?
Because then it may be more economic for me to switch to DC coupling and use Multiplus II units (a bit cheaper). I guess I would have to apply some magic pouder to start the generator if the power-grid if off and the battery below some percentage and stop it onces the grid comes back or the battery is above a certain percentage.


Would be a good feature if one quattro could handle all three phases like the Fronius Gen24. Three Quattro are more than twice as pricy as one Gen24.

2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

marekp avatar image
marekp answered ·

@Sesshoumaru

I build first my system on 3 Multiplus-II-48/3000 units in 3 phase formation.

Soon I found out that 2400W per phase is not enough, so instead of replacing those 3000VA units with 5000VA models, I added additional 3000VA unit to each phase in parallel.

Those 6 units are connected to 20kWh LFP battery (Winston) using long brass buss-bars.

System has Fronius 10-M-3 AC coupled and two MPPT (250/70 and 150/35) DC coupled.

6mp-iifr.jpg

I have 6kW DC diesel generator (120VDC/50A)

Charging this battery directly from diesel will be done (I am working on that now) by connecting this DC generator to the charger and charger directly to the buss-bars.

Same way MPPTs are connected.

Victron does not have dedicated LFP DC charger with parameters I need (120VDC input and 48V output), so I may build my own charger.

Automaic starting of the generator, in the situation you need, is simple. It is a part of Victron software.

As for Fronius GEN-24. I looked at it and decided to go with Victron.

GEN-24 cannot be used as UPS because it needs up to 90 second to switch over from grid to batteries. Also it needs an antyislanding unit (not cheap) to be legally connected to the grid.

Victron MP-II has it build in.


6mp-iifr.jpg (384.6 KiB)
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

sesshoumaru avatar image
sesshoumaru answered ·

@MarekP
Impressive setup. Thank for sharing that image


I have a 230V AC generator and I heared in generel that BYD batteries are quite picky when it comes to the charger they kindly allow the grace to load them. So playing it save for me sounds like I need:


A) Switch from AC to DC coupling

B) Use two Quattro VA5000 units for 2 phases (third phase comes later to make it cheaper now)

C) Plug diesel generator as second AC source into the Quattros

D) Put all major loads across the two phases supported by the Quattros and dispensable loads on the unspported phase 3

E) BYD LVS 12 kWh battery will be connected to both Quattros

F) My own 5KW UPS will secure phase 1 for edge cases (very importand senstive loads)


I hope that sounds like a viable plan and finally settles my endavour into backup power supply :)

13 comments
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

marekp avatar image marekp commented ·

@Sesshoumaru

If you are later planing to make a 3 phase system for 3 phase loads, do not wait to long with buying that third unit. For 3 phase system units must be identical.

If you do not have 3 phase loads, units do not have to be identical.

If that system is to be connected to the grid, Quatro is not certified, in most countries, to be connected directly to the grid without antyislanding unit in front of it.

I heard that there will be Quatro-II that could be certified like Multiplus-II is. It could be also less expensive than Quatro_the_original.

I do not like generator connected directly to the AC-in because it has to be a very good quality generator and therefore expensive. Victrons are very picky about the quality of the AC going into them.

This is also why I prefer to charge battery directly.

Make sure that Quatro will accept the AC from your generator.



0 Likes 0 ·
Show more comments
marekp avatar image marekp commented ·

@Sesshoumaru

I noticed that your described your generator as 230V.

Is it a single phase generator?

If it is not 3 phase generator, usually described as 400V, it cannot be used as a source of AC power for 3 phase Victron system.

You can use it to charge battery directly thou. :)

0 Likes 0 ·
Show more comments
shaneyake avatar image
shaneyake answered ·
  1. Yes, you will need 3 units. You don't have to start off with 3, you can do 1 and then grow. Only thing to keep in-mind is that if you ever wanted to have multiple units per phase they have to be identical but you can have same model but diffrent revision on diffrent phases.


  2. You can have single phase into 1 inverter and 3 phase out from the units but if the generator is directly providing 1 of the phases and inverters the others, there will be a synchronisation problem. You either need devices to be powered from Generator or Inverters but not half and half.

  3. Yes, all inverters will be connected to same battery bank, with BYD or Pylontech or any good battery bank there will be a CAN bus connection to the Cerbo to tell the Victron equipment about the battery.

  4. Is the Fronius 3 phase? If so, yes, 3x 3000 is 9000W, if it is single phase then no, won't work.

  5. This doesn't have an effect on the Victron.

    As you have noted in other comments, I think a Quattro-II 48/5000 is best suited for your application. There is a big investment and it is better to spend a little more now than to do an upgrade later and try sell 3 inverters.

    AC-IN 1 goes to 3 phase Genny, Battery Charging will be controlled via CAN bus.
    AC-IN 2 goes to GRID

    AC-COUPLED is good and will work and you have a genny so if you had a completely discharge your batteries you could start up from that. I personally would still install some via DC-coupled up to 50%, just as a backup in case I lost grid and the genny for some reason. Like outage prevents you from getting fuel.
24 comments
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

sesshoumaru avatar image sesshoumaru commented ·
Thanks for the input.

So I would plan for 3 Quattro II (once available), connect the GetSet as three-phase AC source over all three Quattros and rely on the Cerbo / Venus to handle loading and start/stop of the GenSet during power outage. Just for sake or security I would plan for 5000VA units. That sounds like the most safe / common place way to design the system.


I still like the idea of directly charging the battery with DC after something like the Syklla, but i can really find nothing about that. Makes me worry that its too exotic.

0 Likes 0 ·
shaneyake avatar image shaneyake sesshoumaru commented ·

Yeah, they were meant to be launched in September been delayed till December.

There is no real difference in charging from a Quattro/Multiplus vs a Skylla.
They both take AC and convert it to DC. The Inverter has way more control and can deliver the voltage and current the BMS is requesting. You are limited to the Skylla-TG which is the only one that can do 48V and it is pretty dumb, in definite need of an update. Skylla-i is nice but no 48V version. When charging the bank from a Skylla-TG you have no control, it is full power or nothing. I highly recommend that you don't use a Skylla-TG with a CAN managed battery. We do use them at sites with very bad grid but that is a limited use case.

0 Likes 0 ·
sesshoumaru avatar image sesshoumaru shaneyake commented ·
Sounds like good advice. I want to use a BYD or Pylontech Battery.

I guess then its safer to go with loading via the Quattros. Maybe in the future I can add a Skylla TG (II) once improved for switching to direct battery loading. If I am allowed (by government) to install more solar, than I would also add DC doupling, but stay with my AC coupled fronious for now (its also 3phase, its the Symo 6-0-3-M).

0 Likes 0 ·
marekp avatar image marekp sesshoumaru commented ·
Quattro-II is more expensive than Multiplus-II. It is more than 500 Euro per unit.

You can make your system using 3 MP-IIs in 3 phase setup.

Connect your house to grid through Grid/Gen switch.

That switch will alternate AC-in between grid and generator.

When grid fails, system will run automatically from battery, and when battery gets depleted you can turn on your generator and connect it to the AC-in by flipping that switch to gen position. Generator would charge the battery with its full power using all MP-IIs.


0 Likes 0 ·
shaneyake avatar image shaneyake marekp commented ·

This doesn't work reliably. The LOMs detection on the Multiplus-ii doesn't reliably run with a generator. This is the whole reason the Quattro exists. He is going to have 15kva of inverter to a 8kva generator, this has a high likelihood of not working.

0 Likes 0 ·
marekp avatar image marekp shaneyake commented ·

@shaneyake

Can you explain what Quattro has extra and above over the MP-II, other than additional AC-input under relay control?

Why you say that it works better with generators?

0 Likes 0 ·
shaneyake avatar image shaneyake marekp commented ·
You can have different LOMs settings for each input.


So disable when on generator.


Because Victron uses frequency based LOMs can be a huge challenge when using generators.
0 Likes 0 ·
shaneyake avatar image shaneyake sesshoumaru commented ·

Why do you want to charge the battery with a charger? I don't quite understand why you would want to add a charger if you can avoid it? Maybe I am missing something.

0 Likes 0 ·
marekp avatar image marekp shaneyake commented ·
@shaneyake

You said it yourself. LOM problems with the generators.

0 Likes 0 ·
shaneyake avatar image shaneyake marekp commented ·
This isn't a problem when using or quattro as LOMs is turned off when on generator.
0 Likes 0 ·
marekp avatar image marekp shaneyake commented ·
@shaneyake

You really thing that this "convenience" is worth 1500 Euro in this case?

0 Likes 0 ·
shaneyake avatar image shaneyake marekp commented ·
It isn't really a convenience when it doesn't work completely.

A good quality 100A 3phase Automatic Transfer Switch is also around $1000 and once you have done all the wiring and DB install and labour it doesn't end up being that far a part. Victron official way is more expensive but not as much as you think.

0 Likes 0 ·
marekp avatar image marekp shaneyake commented ·
@shaneyake

Again is a mater of convenience.

1. You do not need a "good quality 100A 3 phase Automatic Transfer Switch" to be able to connect a generator when grid is off. You can just go to your switch board and flip the manual switch (I payed 60 Euro fro 250A one). It does not have to be automatic because timing is not important in the home solution. It is different with the remote unmanned locations.

It would be probably still less expensive to turn LOM off permanently and install proper anty-islaning relay to conform to local code.

2. Good quality 3 phase generator is expensive and Victron is notorious at not working with less expensive ones.

This is why charging batteries direct is much cheaper solution. even when we have to buy separate charger for it.

It works with cheap 1 phase generators.

0 Likes 0 ·
shaneyake avatar image shaneyake marekp commented ·

This is why I love Victron, so many ways to do things.


I personally wouldn't install a manual transfer switch. I have things like Freezers and CCTV systems that I never want turning off. In the case that the system runs out of power the generator should automatically kick and run the system while charging the batteries. You could say this is convenience but to me it is more valuable to never lose power.

sesshoumaru has said that they have very import loads that should always have power. ATS provides that.

Ziehl anti-islanding relay is € 800,00 without the contactor
Ziehl anti-islanding relay contactor - € 1.100,00
So yes this would save you - € 340,00 if you use manual transfer switch but ATS is worth it for me.

Skylla-TG 48/25 (1+1) € 918,00 - Limited to around 1.3kw
Skylla-TG 48/50 (1+1) € 1.632,00 - Limited to around 2.7kw

So not really any saving but much lower charge power.


Quatro Setup limited to Generator limit or 15kva.
Can also hold generator at Most efficient point. Can also mix generator and inverters for 21kva if you need more power to run heat pumps or hot water heaters if using 6kva generator.

You can turn off switch as group and you can run a single phase generator. Victron inverters don't like cheap generators correct, especially if frequency and voltage not stable. With LOMs turned off, this isn't a problem with any decent generator like the one sesshoumaru said they have.

I would much rather suggest someone install a system that I know will be reliable and work consistently without any problems.
If I am building the lowest cost option I wouldn't use Victron.

0 Likes 0 ·
marekp avatar image marekp shaneyake commented ·
@shaneyake

All depends how often and for how long grid is disappearing.

For sensitive loads author has a separate UPS.

Also he indicated that the money is in "short supply" :)

I agree that the separate charger is a week point as far as Victron is concerned, but maybe it could be found elsewhere.

0 Likes 0 ·
sesshoumaru avatar image sesshoumaru marekp commented ·
I aim for conveneice as well. I hate it when things are weirdly expenesive, but I am willing to spend a dime more for a proper solution. It has to reliably work and should not be so custom that even Victron would say "Ugh".


Feeding power from a charger, like a Skylla, directly to the battery seems only to fly well with basic lithium block batteries. All things which have a BMS with own will, like BYD or Pyontech, seem not to be guaranteed to work. I also find no information, neither from Victron nor somebody else, than they ever used a Skylla TG connected to a GenSet to load a battery DC-coupled - especially not with a known battery brand like BYD or Pylontech. And personally I am not so much of an eletric expert to start dealing with raw batteries and balancers :/

0 Likes 0 ·
sesshoumaru avatar image sesshoumaru commented ·

This is probably now an outlandish question, but my already existing system has a Varta Element 6 battery with 6kwh capacity. As far as I see from the manual, its an AC-coupled battery system.

Is there a chance to somehow integrate it with the GX Device on Victron side?
I could imagie to put it between grid and victron AC-in ports and it should not matter to the system as it would swallow only exess energy the Quattros would have feed to the grid. But maybe it could also be included on the AC-out side with the PV?

0 Likes 0 ·
shaneyake avatar image shaneyake sesshoumaru commented ·

From what I can tell it will be possible to integrate the Varta Element 6 as it supports modbus TCP but it will require some custom setup and basic programming. This integration looks doable but definitely not out of the box kinda setup. Would be custom stuff. With it all setup, you should be able to keep in on the AC-OUPUT of the quattros so it is still available during outage and only charge when excess is available.

This openness is the true value of Victron.

Can you request the Modbus docs from Varta? I can give you a strategy to implement control then.

0 Likes 0 ·
sesshoumaru avatar image sesshoumaru shaneyake commented ·
I will try to request this documentation.

I am software developer.. so coding sounds fun^^

0 Likes 0 ·
shaneyake avatar image shaneyake sesshoumaru commented ·
Oh sweet, then this will be easy for you. NodeRed on the GX does this type of integration quite nicely.
0 Likes 0 ·
sesshoumaru avatar image sesshoumaru shaneyake commented ·
One more challage to add, I guess.

At least I kinda drilled down that I want to go with 8 Plyontech US2000 as battery. Hope they will play nicely with victron. Once the Quattro II gets available, the build can go rolling.

0 Likes 0 ·
shaneyake avatar image shaneyake sesshoumaru commented ·

8 Pylontech US2000 should be fine if you are staying below 10kw of total load.

I would have a look and see if FreedomWon Batteries are available in your area. 20/16 model can deliver 20kw continuously and are great batteries with CAN bus.

0 Likes 0 ·
sesshoumaru avatar image sesshoumaru shaneyake commented ·
Freedom Won - never heard before.

Seems they have a distributor in Europe, but compared with Pylontech its rather complicated to get stuff. But again I know now a new battery supplier.


I should be well under 10kW. Normal loads in my house are (without cooking) 1-2kW. And during power outage I would surely not switch on the oven full power :) -- common sense I hope.

0 Likes 0 ·
sesshoumaru avatar image sesshoumaru shaneyake commented ·

I have read through the Varta manuals and it seems that the system has a sensor reading in-/out going current and uses this to decide when to charge or discharge. In the Web-UI I see that the PV inverter is also configured via ModBusTCP.


From the documentation it seems like that the communication with the inverter it only to visualize the where the power is going or to dynamically restrict the PV system. Both jobs Victron Quattro (GX) takes over. It should do its main job or charging and discharging without this connection as well.


If I keep the load sensor where it is, then the Varta Element should start loading only when current is "feed" to the grid and start discharging when current is drawn from the grid. Sounds fine with me, even without doing something. During power outage the Varta Element would be a dead fish in the water, but that is acceptable.

0 Likes 0 ·

Related Resources

Additional resources still need to be added for this topic