I’m currently designing a Victron Multiplus II based system for taking my house off-grid. I am tossing up between a couple of options:
1 x Multiplus II GX 48/5000 or
2 x Multiplus II GX 48/3000 in parallel
In both cases with an LFP battery bank of at least 15kWh. An existing PV system (output limited to comply with the factor 1 rule and battery size minimum) to be AC coupled. Plus potentially some new solar connected to the battery through Victron MPPT chargers.
Advantages of the first option:
slightly lower cost
slightly simpler design
Advantages of the second option:
higher total output (6kVA continuous instead of 5kVA, 11kW peak instead of 9kW)
redundancy, if one Multiplus ever is out of order, the second could (with consciously reduced loads) keep the most important parts of the house running until a replacement is installed.
Questions:
What’s the likelihood of a Multiplus II failing in the first 10 years? Does Victron publish statistics on that? That will determine how much I value having the redundancy of option 2.
Is the 2 Multiplus in parallel system as easy to implement as it seems (just software settings in the GX device), or does it entail other issues that I might not be aware of? I know I can only have one GX device connected at a time, so I would disconnect the second GX and keep it sitting there as a ‘backup’.
Inverter failing due to poor install. High. I see many failed in the repair workshop for various (install) reasons.
Inverter faliure when the install is done correctly. I have had zero. And have been in the (Victron) business since before covid.
No don’t parrallel. Very inefficient and actually increase your chance of terrible install related headaches. The wiring has to be on point. Among other things and the numbers on paper are not what you will see in reality unless you are pedantic.
If you want a bit more power with greater efficiency use a Multi RS. (Unless you have a grid code thing you have to adhere to). Or an inverter RS if you are really serious about offgrid (i have one, love it).
As a beginner, avoid the parallel setup. One larger system is better in every way and far easier to get right.
My personal system is parallel purely because it grew over time, my others, I have done away with parallel setups when changes were needed.
Edit: To add, the only failures I have had, were on parallel systems, both input relay failures. My theory is the timing of relay operations can be tricky with multiple systems and a bad grid.
Thanks for pointing me towards the RS units. Looking at them now.
Did I see that right, the Inverter RS doesn’t have a generator input? That would rule it out. I want to use an EV with V2L for backup power in case the battery runs low, based on the last 2 years of consumption I would need that about 1 or 2 days a year to comfortably get by with the 15kWh battery without needing to shed loads (which in winter would be heating with AC).
The Multi RS 48/6000: it shows “Maximum operational PV input current limit of 13A”. Is that the current per tracker, so I could have 2 x 13A, or is 13A the overall total? Compared to the Multiplus the Multi RS is surprisingly light, at 12.3kg vs 30kg, how can that be? Did they do a transformer-less design?
I would not need the input relay very often, but when I need it, it has to work.
If the input current that needs to be passed through is a lot smaller than what one Multiplus can handle, would the other one even need to be connected to the input at all?
As mentioned in the reply to LX, I would take things ‘off grid’ and the input would only be used on the rare occasions when a number of cloudy days in a row mean I need to supplement my house battery by using an EV in V2L mode. That would limit the input to 15A.
The transformer-less design rules it out for use with the existing panels. What I have are the old SunPower panels that need to be positive earthed. The plan was to keep them going via MPPT chargers direct to battery once the old inverter dies.
Which poses an interesting question: does positive earthing of the panels mean that the battery is positive earthed too (through the MPPT)? And would that cause problems I have not considered? No need to reply, I usually try Google first before asking in Forums, I don’t want to waste people’s time. Just sharing my thought process.
“Transformerless” means it doesn’t have a massive heavy 50Hz transformer inside to step up the low-voltage PWM (FET bridge across battery) to 230Vac, but it’s still very likely to have a HF (20kHz?) transformer inside to provide isolation between the DC and AC circuits.
I don’t know whether it is true for the Victron Multi RS.
But I do know that other brands of inverter are not suitable for panels that have one pole earthed (not the frame, but in the case of the old style SunPower panels the positive output of the panels).
When SMA went from the SB1100/1700/3800 to SB3000/4000/5000TL they were no longer usable. The last inverters standing I am aware of were the Fonius Galvo series (now no longer in production).
Victron seems to have one perfect drop-in solution for me though: SmartSolar MPPT RS 450|100. It would run the existing panel layout unchanged. The data sheet states: “Full galvanic isolation between PV and battery connections provide additional overall system safety.”
Since that feature is specifically mentioned for this MPPT, but not for others I tend to assume that is the only option I’d have. (i.e. othwerwise I could have used a number of Smartsolar MPPT 100/20 if I rewire my panels from an 8S into a 2S configuration).
