I’ve read about Fronius as input to Multi/Quttro inverters and was wondering if as an alternative, could one use a Multi as an input to a Quattro eg. on the Generator input?
I’m in the fortunate position that I have a Multiplus II and a Quattro 10k (long story) - Quattro is my main inverter and if this can work without blowing something up, would allow me to add another 5/6k PV and some level of additional battery to my setup without having to rewire the house
In my opinion, this won’t be possible at all, the only way to get more capacity with Victron inverters will be to add a Multi / Quattro of the same model in parallel with the first.
Do you actually need more inverting capacity than the 8 kW the Quattro gives you, this is not so clear from your post, or do you simply want to add more batteries and then more PV to charge those extra batteries?
You can add more PV and batteries to your system without having to add another inverter.
Alternatively if you need more inverting capacity it might be worth looking at selling the Multi and getting a Fronius, this might just be the most cost effective route rather than having to add a second Quattro in parallel.
@Sarel.Wagner told me in his microgrid thread that the Quattro and Fronius are not so happy to work in parallel should you need extra inverter capacity, but in one of the Victron training videos they actually list it as a advantage / feature of having a Victron and Fronius in a microgrid setup, that they can work in parallel to increase your inverting capacity.
Thanks for your thoughts… have plenty Inverter capacity
Planning to go off-grid, would like more PV for overcast days and also planning to add extra battery for some “breathing room” for night time loads and yes, am aware that I can add to existing system. The Multi can also serve as a backup if I need to take the Quattro offline for any reason…
Not ruling out that option…
Thanks again
Makes sense. Sell the Multi and get the right tools for the job.
Fronius is the right solution, not a Multi on the input. You can have a Fronius (there are other brands as well) either on the Utility mains input, or on the Quattro output. I prefer it on the Quattro output.
Yes it is a bit sensitive the Fronius, but only if there are no Utility grid (the grid is stiff with Utility input act.ive). The Fronius will disconnect when there are really large load switching happening and Eskom is not active on the input of Quattro. It recovers in about a minute or so and then works well until the next big load swing. I find that mostly with my geyser switching on that I get the disconnects ( geyser =4kW). Otherwise its stable.
Groetnis
Hi @SolarSmurf , I can not see why the inverter will allow for more PV power. The PV connects to a MPPT and batteries can always be added to the quattro.
The Quattro can only use one of its inputs at a time, and depending on your setup it would choose input on or 2 as its default input. You would connect the Multi to the Quattro, but the panels and batteries connected to the Multi will only come into play what the grid fails. (Again depending on your config… )
Late to the party, but by accident I have a 3kVA upstream from another 3kVA. The socket in my office, where I have my test rig, happens to be on a circuit that is backed up by another Multi. As long as the grid is on, it works okay. The two Multis can both feed energy into the grid and all is stable. The moment you go off-grid, things are less well. The downstream Multi is not at all happy with the quality of the AC of the upstream one. There is no way to make this sort of setup work reliably in an off-grid setup.
Thanks Gents for all your thoughts and comments, will add the extra PV and Battery to the Quattro for now
I did realise after my post last night, that this relates specifically to having ESS installed on the downstream Multi. Mileage my vary if the downstream Multi is not attempting to “tie” with the upstream one. But that is what you wanted to do, right? 
Hi @plonkster, adding a bit more detail to my idea…
Upstream is an off-grid Multi feeding downstream to an off-grid Quattro with ESS.
Multi output is connected to “generator” input of the Quattro - the Quattro input is configured with LOM detection disabled
On a sunny day, the Quattro handles the loads/charging without a sweat, the Multi will be doing very little other than maybe charge it’s battery if required
On overcast days, the Multi will supplement the Quattro, assuming there is not enough PV on the Quattro to meet demand - Quattro already has 3 x 100A MPPT’s (East, North and West) so i’m a tad nervous about adding additional PV to the Quattro…
At night (overcast days), the Quattro ESS “Grid setpoint” and “Max inverter power” are set at a “number” that will handle the base load and that short peak loads (Microwave etc.) are catered for by drawing from the Multi - Multi is acting as “Grid” for the Quattro
The main goal is to have the additional PV on the Multi for overcast days to supplement the Quattro and minimise the use of an actual Generator
Will still need the Generator for really dark, overcast rainy days
Anyway, that was my thinking and I thought i’d get some ideas here before I asked someone to hold my beer… 
Right. So my original assessment was accurate then 
You don’t add pv to the Quattro, you add it to the DC bus… If you have enough battery’s to store the power you can add 100 x 100A MPPT’s without a problem.
You would also need batteries for the Multi, you could rather add those batteries to the quattros bank and have “All day, everyday” benefit from them… Adding them to the multi that will only work occasionally, effectively means that you wont benefit from tht bank most of the time, it will just sit there waiting to do something while you could use them more effectively in the existing bank, reducing the need for a generator in any case.