warly avatar image

Advice on off-grid 2 houses 4 x Multiplus installation options


I am designing an off-grid system for 2 houses based on AC-coupled PV inverter and Victron Multiplus.

I need about 12 KW total power to cover yearly electricity needs. The houses are located in Ireland.

I am wondering what is the best design, I am thinking of 2 options, having one central power generation system, with 4 Multiplus in parallel and the 2 houses connected to AC1, some dump-load on AC2, or, another option, keep the 2 houses mostly separated for redundancy, but still have one AC2 of the first house feeding in the Ac-in of the second house to have some back and forth power surplus flowing up or down.

The 2 houses have already a 6mm² AC wire between them for historical reason, I cannot add a new one.

See the diagrams for more details:

Option 1: 2 houses on a central power system

Option 2: 2 houses separated

There may be better other smarter options. At the moment, I have already a MP2 48/3000, this is why I was thinking of reusing it. But having 2 x 48/5000 is maybe a better option. Similarly, there is already a 3.6 KW ac-coupled PV array on one of the house, this is why ac-coupled seems a better option.

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2 Answers
warly avatar image
warly answered ·

Thank you for your answer jbakuwel.

Regarding the battery cable sizes, I tried to look at various recommendation for the continuous max current (100A per battery, 50A per MPII), but maybe I should reconsider if you think this is not enough.

You are right about the 1:1 rule and the house 2 potentially overloading the house 1, so feed-out power from house 2 probably need to be limited.

You are right about the mis-named "dump load", this is more like non-essential load that would use power excess.

Good point about LVD event, I'll think about that. Limiting the PV output at the inverter level is a good solution to prevent overload, I may oversize my panels for winter, indeed.

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jbakuwel avatar image
jbakuwel answered ·

Hi @warly,

I am not so sure about your battery configuration for option 1 but have to say I haven't yet looked at what the Lynx boxes can do. Are you sure your battery cables (16mm2 and 25mm2) are as per the requirements (see MP-II documentation)? Looks a bit marginal to me.

An issue with your proposed wiring of option 2 is that once house #1 starts feeding into ACin of one of the MP-II's in house #2, you are going to exceed the capacity (violate the 1:1 rule) of the other 3KW MP-II with your 6KW of AC coupled PV inverter(s). More about that here.

What is the purpose of the "dump" load? Dump loads are required for wind generators as these can't derate themselves sufficiently but are not required for PV systems.

I am in a similar position (linking two buildings) albeit with different requirements. Yet what I'm looking at might work for you too. There are still a few unanswered questions but as far as I know what I have in mind works just fine. You can read more about my design here.

Redundancy is one of my requirements and I'm still working on the exact layout. 1-0-2 switches can potentially be used to feed power from building #1 to building #2 or the other way around (never at the same time of course). There are hot water cylinders too but these are not named "dump loads", just yet another power consumer (electric cooking/oven, electric car being other ones). These loads do not all need to be on at the same time, so I'm not trying to size the power system to accommodate the max theoretical load but instead use a smart control system that prevents the total load from exceeding the available capacity of battery and PV inverters. For example that control system can stop heating the water when kettle power is requested and resume heating when the kettle is done. The complex part of this control system is the varying power production of the PV inverters but that topic is outside the scope of this post.

You should also consider adding a MPPT charge controller for each battery bank so you can bring the batteries up again after a LVD otherwise your power system will shut down when the LVD kicks in and will never recover from that.

Last but not least it's perfectly feasible to oversize your solar arrays and rate limit your PV inverters to not exceed the capacity of the (combined) battery inverters. The main reason for this is to ensure there is enough solar power generation capacity in the winter. In summer you'd have excess power generation capability which you won't (need to) use.

Hope this helps,

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