Coupling Options For Off-Grid Solar Arrays

This post was originally published in 2012. The tips and techniques explained may be outdated.

Off Grid DC Coupled System

Substantial improvements to off-grid photovoltaic technology during the past decade have led to more choices in off-grid PV system design.

Installers can choose between direct-current (DC) coupling with a charge controller and direct alternating-current (AC) coupling of an off-grid or grid-tied inverters to the AC bus for these applications. The appropriate coupling method varies by project.

Off Grid AC Coupled System

Advantages to Both Methods

Both AC- and DC-coupled systems provide a renewable energy source where power normally is not feasible, but there are advantages and challenges to both methods.

Traditional off-grid systems are DC-coupled. The battery bank is charged by connecting solar modules to a large charge controller, which regulates both the voltage and current.

With AC coupling, an AC-synchronous solar inverter is directly connected to the AC loads panel. The DC battery bank powers the DC-to-AC inverter, with solar production fed to the AC loads panel. Any extra power on the AC bus is converted to DC to charge the batteries.

Find real-world examples of off-grid AC-coupled and DC-coupled systems, and learn why AC coupling could offer more advantages, in the September 2012 issue of Solar Industry magazine. “Coupling Options For Off-Grid Solar Arrays: DC Versus AC Setups” begins on page 48.

97 Comments
  1. Avatar
    Stephen More says:

    I currently own a SB7000US that is grid connected. How can I economically use my solar array to power my fridge during a power outage ?

    I believe the Sunny Boy 7000-US is too large for a Smartformer, so is the only other option two buy 2 sunny islands ?

    Reply
  2. Greg Smith
    Greg Smith says:

    Hello Stephen. “Economically” is always in the eye of the money holder, however using 2 Sunny Islands to get a 240VAC output is a bit much. You are correct about using your SB7000US with the Smartformer being incompatible (unfortunately) but there are other solutions out there. All you really need is an auto transformer that can handle 56A of AC current flow. remember, if it is 56A Line to Line back feeding through the autoformer to the Island, you will have 2x the current since the voltage will be 1/2. AEE sells autoformers that will work and I even found one on Grainger’s website that will work as well. Midnite Solar will be introducing a compatible device too.

    If you just want to power your fridge and a few other low consumption loads then I think the most cost effective solution would be to buy a 1kW Honda generator. Bring it outside during a power failure and run the loads directly off it it.

    Reply
  3. Avatar
    BriAN says:

    I have a grid tied 1.7 kW PV system with a SunnyBoy 3000 – US. What will I need in terms of using an SMA charge control for a grid tied battery backup system. I also have a manual transfer switch for my generator. Please advise. Thank You.

    Reply
  4. Avatar
    Manny Rentetia says:

    I have a client that will be disconnected from the grid soon and will need to convert
    To off-grid self consumption, if that term even exists.

    The average monthly kWh consumption is about 900kwh. Assuming no grid will be connected (for now) and possibly reconnected in the future.

    What would be the most cost effective initial solution for her? Initial system ideas/quoted

    (16) 315 w solar world 72 cell
    (1) 6048 sunny island
    Batteries?
    Grid tied inverter or charge controller?

    She has a few 220v lines (dryer, stove, water heater). She hates gas appliances as she’s recently had surgery due to carbon emissions..? Yeah I know…

    Can someone please help!!?
    Manny@cmse.me

    Reply
  5. Avatar
    Steven Kong says:

    I wonder if the tripower inverter can be compatible with the Multicluster.
    The tripower has the output voltage 277v; the Multicluster has the input and output for 120V.
    So if using the tripower inverters with the Multicluster and off grid inverters, there should be at least two additional 120V 277V transformers needed.
    Am I right?
    Thanks
    Best
    Steven

    Reply
  6. Avatar
    Carmen says:

    I’m looking into powering a swimming pool pump from a PV array, and having the pump run only when there is sunlight.

    Could I implement a Sunny boy inverter (say the 1300TL) to connect the PV modules directly to my AC load without connecting it to the grid nor batteries? (I live in an area where grid-tie is not implemented yet)

    Thanks

    Reply
  7. Avatar
    Gavin says:

    Hi There,
    Is it possible to get the best of both worlds and have both AC and DC coupling through the Sunny Island? For example, a 4kW array through an MPPT controller direct to the batteries, and 10kW through an SMA SMC10TL?

    Reply
  8. Avatar
    Harsh says:

    Hi, How come I decide specifically whether I should go with AC coupling system or DC coupling system? I would appreciate if you illustrate it with an instance at earliest.

    Thank you in advance.

    Reply
  9. Avatar
    john says:

    I have a sunny island 8.0h and three sunny boy 5000TL units with 15kw of solar panels in 5 arrays.

    My battery bank is currently 8 banks of 48v (12v AGM batteries in a set of 4)

    is this a sensible battery config or do i need to address any balance issues?

    Reply
  10. Avatar
    Doug says:

    We have a Sunny Boy 500TL-US-22 that is currently grid-tie. We want to add a battery backup system using a Sunny Island unit. Our monthly avg usage is approx. 500 kwh. Which Sunny Island unit do we need and what battery system do you suggest. We are below average energy users and very conservative relative to heating and cooling use in our area.
    Thanks,
    Doug

    Reply
  11. Avatar
    Stephen More says:

    Its been a few years since my original post “I currently own a SB7000US that is grid connected.”

    I see the Sunny Boy Storage 2.5 will be compatible with the Powerwall and can be AC coupled.
    1. When will the Sunny Boy Storage 2.5 be available in the United States ?
    2. When it is released, can it be AC coupled to the SB7000US during a grid power outage ? ( if so it would be great to see a high level diagram of the major components needed ).

    Reply
  12. Avatar
    Tim Fitz says:

    Our church just purchased an off-grid retreat site (formerly owned by a school district) and they have in place a 3 phase system system that has been getting power from a diesel generator running 24/7. Needless to say, gas is expensive, this is was too dirty, and we want a better solution. The problem is that we can’t break the bank.

    Usage: average usage is about 100kWh a day, but can peak to 400kWh a day on busy days
    Power source: 75kVa (56kW) diesel generator producing 3 phase
    Architecture: 3 phase micro-grid that feeds about 15 structures
    Current thinking:
    – first phase: get a battery system so we can have generator run for only 2 hours a day (to produce the 100kWh) to charge the batteries, then run off batteries between cycles.
    – second phase (and subsequent phases): as we have budget, install PV cells on structure roofs incrementally and tie them into the grid.

    My thoughts:
    – seems like we need at least 3 Sunny Island units wired up two give 3 phase power via 48V batteries (Lithium Iron perhaps?). These will turn on the generator when needed and provide the “micro grid”
    – one option: get 9 Sunny Island units in a Multi-Cluster box and get 56kWh of 48V storage and have them all in a shed next to the generator. Downside to this: Sunny Island units are expensive and 48V storage is not as economical as something in the LiION range (what the Sunny boy storage will handle)
    – second option: get 3 Sunny Island units, no multi-cluster box, a small amount of 48V battery (18kWh). Then we buy 15 Sunny Boy Storage units and connect them to LiIon batteries. Downside: many more devices/boxes. if we got Tesla Powerwalls @ 6.4kW per unit (one for each Sonny Boy Storage), then we get 96kWh total storage. cost might be economical though… major downside: finding tesla powerwalls is impossible

    Any thoughts about the advantages/disadvantages of either way?

    final question: when are the sunny islands going to be able to go above 48V DC? (to connect to a powerwall)?

    Reply
  13. Avatar
    linda verwilligen says:

    To achieve an off-grid power source of 250kva using Multicluster Box,what brand of Multicluster Box will suffice?How many Sunny Island will be needed and invariably how many Batteries? Kindly revert.Thanks

    Reply
  14. Avatar
    Danny says:

    Hi,
    Can we use Sunny Tripower instead of using 3x Sunny Boy for AC coupling 3 phase? However it must be 3x Sunny Island, right?
    Thanks
    Danny

    Reply
  15. Avatar
    Thorbjorn Waagstein says:

    We have a hotel in Nicaragua with a 4.2 kW solar array (with battery backup), which covers around 7-8% of our electricity consumption. As we have abundant sun, high electricity price and no feed-in tariff, we have planned to expand our system with 46 kW solar and a big Ni-Fe battery bank (120 usable kWh), which should cover 80-90% of need. Our system is split-phase 120-240. Our present system is set up with a Xantrex inverter and a Midnite Classic charge controller, but I wonder if we would be better off with SMA, as the max array of Xantrex as I understand it is 6×6.4 kW inverters. The system we want to setup could perhaps be described as “Off-grid with grid back-up”. My thinking was to keep my present battery backup for when my new battery bank has been discharged in the late night hours and the grid goes down, so I can keep my prioritized circuits running in these unhappy (and rare) circumstances.
    What would be the recommended set-up be? 7 SMA 6048US and then a number of charge controllers? I would not want to charge the new battery bank from the grid, only form the solar arrays.

    Reply
  16. Avatar
    Wes says:

    Of course it is possible to use the sunny boy to power a sunny island in a grid down situation, but my question involves an onsite backup generator.

    My 12kw genset makes 220v via a manual transfer switch and when I run the generator, I want it to charge the sunny island batteries. There is a switch that senses the position of the transfer swtich back to the sunny island’s input. When the grid is down and my genset is off, I’m fine using the 120v output of the sunny island for my critical loads.

    Will this setup work?

    Reply
  17. Avatar
    Baneeshya says:

    We have purchased a Sunny tripower Grid tie 3 phase inverter STP10000TL. Wish to convert it to off Grid or Hybrid. Is it possible?

    Reply
  18. Avatar
    Jaco Strauss says:

    Dear Bill
    I need some tech support regarding the sunny island inverters
    I have a sunny island comisioned with the new tesvolt lithium Batteries
    I comisioned the system today and I had an earth leagage tripping all the time on the load side off the sunny island.I bridged the Neutral and earth on the utility side but still that havent resolve my issue.I then also bridge the neutral to earth on my output side off my sunny island.That stoped the tripping of the earth leagage.The system then operated for about 6 hours with a sunnyboy 5000 .Then suddenly the inverter start making a humming sound.It still produces power but a loud humming sound which im not familiar with.almost like if the inveretr are under alot off load but thats not the case.I then shut it off.Any sugestions.please reply to my email

    Reply
  19. Avatar
    Michael says:

    We invested in a Tripower 8000TL unfortunately the grid operator has been unable to supply us with the promised 3 phase supply and as a result the inverter has been idling now for over a year. We do have a single phase supply. Is it possible to use the Tripower with a single phase? As long term solution we are looking to take the Tripower off grid. We will purchase a 10KV 3 phase generator as well for those rainy days. We are looking for a solution where the generator, the Tripower and off grid solution all coexist. Be grateful if you could pass this request to the application engineering team.

    Reply
  20. Avatar
    Steve says:

    Can i make up my own battery bank of 240vdc with a current limiter and and feed it in one of the solar inputs of the tl 5000 sma inverter. ..if not why?

    Reply
  21. Avatar
    Miguel says:

    I live in the Caribbean and have a solar array of 30-270w (8.1k) panels and a SMA 7.7-US. I would like to go for an alternative off grid system due to our quite frequent power blackouts. What kind of Sunny Island would you recommend and what other additional system/devices do I need to charge let say a bank of 12 batt to feed my home?

    Reply
  22. Avatar
    miguel says:

    I meant the necessary step TO ADD to my array an off grid alternative system just in case of earthquake, hurricane, etc. The Puerto Rico disaster experience calls for an essential and preventive alternative autonomic system for us in Dominican Republic.

    Reply
  23. Avatar
    Wayne Hartlerode says:

    Just purchased a SI 6048. Will the SB5000US work as an acceptable pv inverter and if so do are there other accessories required to achieve AC coupling besides basic wiring?

    Reply
  24. Avatar
    Mohamed Didi says:

    I have SB5000TL inverter grid connected. I am forced to add one Sunny Island SI 4.4 inverter with batteries to the existing system, as my utility provider is not interested to allow me to export my excess production to the grid. In return my plan is to use grid power as less as possible. But I need to arrange a setup so that is will be fully automated, having PV power at daytime, after the sun goes down start battery and when the is drained to the DOD, start my generator which must charge the battery and continue. In practice, utility power is secondary and a backup. Also, if the utility power goes off, the PV must continue during the outage. My question is what kind of automatic transfer switch would be required or with such a switch, is it possible.??

    Reply
  25. Avatar
    Dan says:

    Hi
    I got 2 sunny island 8 working off the grid with 2 sunnyboys 5.
    28 x 340 w pv
    And 48 v. 1850a pzs batteiies.
    At the beginning I had the sunny island working
    And from time to time at night a would have a fall in the % of the batteries and fall of the power.
    From 70% to 25% very quickly.
    I had installing a shunt to the inverter even tough everybody told me I don’t need one and that I got a problem with my settings.
    After the shunt is there it’s working great, but now the % isn’t leaving the 100%.
    1. Do I need a shunt?
    2. If yes, and I got 2 si8, do I need 2 shunt?

    Thank you

    Reply
  26. Avatar
    Sindre Ulfsnes says:

    Hi,

    I am designing a large energy system for an off grid location. The system will be completely off the grid. The PV array will be 130 kW+ and battery bank will be around 130 kWh. I had e general question about AC/DC-coupling for designing such a system. Is DC coupling even relevant? I’ve only found charge controllers that operates with a nominal battery voltage of 48 V. With such a large battery bank I am assuming that it would be better to AC couple the system with solar inverters and inverter/charger?

    Reply
  27. Avatar
    Peter says:

    Where I am, BC Hydro (local utility grid) is likely to get rid of net-metering (yes they are crazy). So instead of 27kw pure grid tie, now I am looking at having 3phase power in my barn which can run machinery. I would have a lower power 120/240vac 60A service coming in but mainly as a battery charging source during winter. There is no 3 phase grid in my area. 1 or 2 large 48v forklift batteries for storage. So now looking at 15-25kw solar input (tripower?) and I’m assuming I would need 3 of the sunny islands? For 3 phase it seems a lot of hardware (3 islands) vs one large inverter for 3phase generation. I would also include generator backup. A 1-3kw wind turbine as well. Any advice? The online web system designer is just too confusing, tripower can’t even be selected. I’m in Canada.

    Reply
  28. Avatar
    Tim Tucker says:

    If I add additional emergency dc disconnect between the solar panels and the supplied dc disconnect, would this effect performance in anyway ,as for fire department to access the disconnect from outside in emergency,,the sunny boy 3000 will be mounted in doors.

    Reply
  29. Avatar
    Diamond J Legend says:

    Hi All,
    What is the setting in Sunny Explorer to configure for off-grid?
    Is it “Island Mode (0)”..?
    FYI – I have an off-grid system with both AC & DC Coupling using a Victron 150/35 MPPT, 2 x Victron 48/3000 Multigrid in Master/Slave, a BYD LV Battery bank and the SMA Sunny Boy 5000tl-20 in question.
    Your assistance would be greatly appreciated because at present I have to turn the AC Coupled inverter off each morning before the batteries get to 100% or I lose power when the Multigrids switch off at that point to prevent damage to the batteries from overcharging.
    Damn annoying if I forget and everything gets reset..!!!
    Cheers!
    Diamond

    Reply
  30. Avatar
    Nosakhare Osaretin Paul says:

    Hello Bill Alvarez,
    Please, I am working on a design for AC coupling system for a system can work only during the day (9:00am to 6:00pm), with a battery bank with autonomy of 3hrs. I want to power a load of 18kW, and I want to use 3 SMA Sunny Island of 8.0H, and Sunny Tripower of 20kW and a Renewsys PV Modules of 300wp.
    Please how many PV Modules do I need to achieve this and how may battery bank?

    I want to use Lithium-ion batteries and i’m in Nigeria (Lagos).

    Reply
  31. Avatar
    Uttam Chauhan says:

    Hello Mike,

    I am looking to add SMA to SunnyIsland 6.0 and need a list of compatible SMA / SMA TL Inverters which are compatible with it. Can it be SMA 6000 TL / SMA 6000US-22

    Another question can we do AC coupling with other inverters other than sma or use Enphase for AC coupling?

    Thanks!

    Uttam

    Reply
  32. Avatar
    Wes says:

    These questions were asked but never really answered. So the only way the Sunny Island 6048 can operate without the TL inverters is with a charge controller on an off grid situation. Or can the SI accept PV voltage and charge as well? Also as another person asked, Can this operate in grid mode with other inverters, for example Enhpase?
    Thanks

    Reply
  33. Avatar
    Antonio says:

    I have a sunny boy 5.0 US split phase (two 120V lines) connected to the grid. How I can add battery backup? What is the recommended setup? Is the sunny Island an option?
    Thanks

    Reply
  34. Avatar
    Keith Bacon says:

    So I am having trouble getting my si-6048-us-10 to recognized my generator’s input.
    I am/was using a Honda EU 2000i generator. I connected it all up properly and couldn’t get it to sync with my Honda. After a very long and dark night of reading the manual with a head lamp (the generator was running just no power was syncing with the inverter), I went to add gas and noticed something had back fed my generator and fried the inverter on the Honda.
    I am not putting blame on anyone because I was the one doing the work on my own system. And I have a honda eu2000 graveyard with enough extra parts that it is no more than an inconvenience having to swap the broken part out.

    So my question is what would of cause this to happen?

    I live where there is no grid power.

    I also don’t understand how to force charge the battery if I have a battery needing charge but is to low to start the si-6048-us-10

    Thanks for the help understanding.

    Reply
  35. Avatar
    Pencho Genov says:

    I already bought a Sunny island 8.0-13 inverter and was thinking of using mppt solar charge controllers for my batteries. But now I think that maybe AC coupled system may be better alternative. If I buy 2 sunny boys 6.0 inverters for my 20kw solar array can I use the combine power of 18Kw (from the tree inverters during the day when PV is producing power? Thank you!

    Reply
  36. Avatar
    Peter says:

    I’m working on installing a 26kw grid tie (2x 7kw sunny boy, 2x 6kw sunny boy) and two SI6048’s 120/240 split phase. Anyhow the 2x 6kw inverters will be on the critical loads panel as the 6048’s can’t handle more ac coupled/grid tie power which is fine. My question is, I have a 2000w hyundai HY2000si 120vac inverter generator – can this be hooked up to only one of the sunny island’s or do I need a 120/240vac split phase generator only? I do have a 4000w bigger generator but it’s very noisy. Chances are I will rarely use it with the 12kw of ac coupled solar. I pulled apart a nissan leaf 24kwh battery which will be reconfigured to 48vdc for the sunny islands, but still on the fence as to which bms I’m going to use.

    Reply

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