petr-klus avatar image

AC or DC coupled install


I am in a process of designing the basis of my solar system, and I have a question about ACvsDC coupling.

I am considering a setup where around 50% of the energy should be self-consumed immediately, with the rest charging the batteries or being sold back into the grid if the batteries are full (which may be often during sunny days).

I am considering Multi/Quattro as the basis of my system, with at least some of my capacity going via MPPT solar chargers directly into the batteries, to avoid any deadlock situations.

My question is - shall I just pump the whole capacity via the Victron MPPT solar chargers and avoid a separate AC inverter? Is there any additional strain on the batteries if they effectively share the bus via which the energy will be going through the inverter back to the appliances/the grid?

AC PV Coupling
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I think this would depend on what size system you are planning on having. From what I have seen cost wise if you are going with more than 3 Victron MPPTs then you should switch up to a Fronius for the AC coupling. But you would still need at least one Victron MPPT direct to the battery for autonomous system recovery.

I am looking at between 5-8kWp, how can I calculate how many MPPT's would I need?

EDIT: Looks like my setup should be catered for by two 250/100 SmartSolar chargers.

So back to my question - is this setup more detrimental to the battery in any way compared to having only a single smaller charger + AC-coupled large inverter?


If you believe you can consume 50% of the generated energy immediately, I would advocate for an "AC arm" in the installation. A small Fronius Primo (assuming you have a 1p system) 3.5-1 or so should do the trick.

Using that energy is more efficient than going the MPPT -> Quattro -> AC way.
You avoid the deadlock with having a "DC arm" with one 250/85.

I also suggest to overpower the PV fields (under dimensioning the inverters/MPPTs) to have a better summer/winter balance.


So the answer is "not AC or DC, but AC and DC". ;-)

With an 8KvA your needs will be taken care of with 2 x 250/100. We have set up several 8-10KvA Victron with these two in the system with 20x 390W panels (filled the roof). We are not allowed to feed in to grid here but I can say that it would definitely by 10am, their 16KvA batteries are fully charged easily by then. And the Victron inverters can be programmed to export to grid with ESS without draining your batteries feeding into the grid.

1 Answer
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wkirby answered ·

I would go for a DC coupled system, it's simpler. With AC coupling you'd need to observe the 1:1 rule, you'd need at least an 8KVA Quattro if you want to AC couple on AC out.

I like to think of AC coupling as agood solution for incorporating existing PV systems into a newer battery system.

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Please excuse my naïve understanding - if I go this way, would this somehow more "strain" the batteries by virtue of them always being on the same bus through which all the power will flow?

(I am considering the BYD B-BOX 13.8 as the battery)

I think I understand what you mean.
The MPPT chargers will not allow the DC bus and therefore the battery Voltage to go above the limits set within it.
A large amount of current flowing through the DC Bus between the MPPT and inverter during the daytime means there is less current flowing from the battery, if any. It will hopefully be charging the battery as well as powering the loads.
If you are thinking about high battery current (strain) then early evening time will be peak time for this. The sun has gone and the family is busy enjoying cooking, mealtime etc.

The BYD batteries also have additional layers of protection within themselves.

If Petr goes with 8kWp (plaster every space with PV you can get), he can easily have a Quattro 5kVA a Fronius Primo 3.5-1 and a 250/85 MPPT and adhere to all rules there are.


How would this work, are you suggesting to have the AC-coupled inverter completely separate from the Primo? How would it be wired for me to not have to observer the 1:1 rule and would this also be active in case of grid failure?

Put the AC coupled on the AC out.

Put the DC coupled on the bus bars. This way the DC can help you to recover the system is the battery goes flat - black start.