Coupling Options For Off-Grid Solar Arrays

From on 10/11/2012 in Category Technology with 46 Comments
SMA Inverters - Sunny Boy and Sunny Island

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.

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46 Comments

  • avatar

    Stephen More

    10/26/2012 at 11:54 am

    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 »
  • avatar

    Greg Smith

    10/29/2012 at 11:59 am

    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 »
    • avatar

      Stephen More

      11/08/2012 at 5:02 am

      According to my Sunny Explorer, on 3/28/2011 my SB7000US was pushing 7.1 kW for 3 hours. The Sunny Island has a Max. input power of 6.7 kW. If I had 1 Sunny Island operating on 3/28/2011 would my production throttle down to 6.7kw or would the system just shutdown completely ?

      Reply »
      • avatar

        Greg

        11/08/2012 at 2:38 pm

        You can only push through 56A from AC1 to AC2 of the Sunny Island. 7000W/24V = 29A so you would be ok.

    • avatar

      Scott Eliott

      02/11/2013 at 8:25 pm

      We have a similar issue to Stephen More. I recently invested in a Sunny Island 6048 with Smartformer, and this is being added to an existing SB7000US grid-tie PV plant installation. The installation guy saw that the Smartformer will work with the Sunny Boy 7000US for non-grid tie, and procured it based on his assumptions. I have several questions about this:

      1) Assuming I had a SB6000US, is there an existing circuit diagram for a split-phase gasoline generator, SI, SB, and Smartformer that more precisely shows a recommended configuration of both an auto transfer switch, as well as a manual transfer switch? The diagrams in the SI and Smartformer are generic and it would be good to understand a little more of the “why” the differences between the two approaches, other than the obvious split-phase operation.
      2) My system currently has the PV plant directly tied to the grid, but with the purchase of the new equipment it is desired for the PV plant to be connected to the Sunny Island and Smartformer during an outage, but to the grid the rest of the time. What is it about non-grid that makes the SB7000US more compatible with the Smartformer? One gets the impression that appropriate switching could enable a non-grid situation for the SI + Smartformer, such that it could be used with the SB7000US. Is this not an option, and what functionality of the SI would the user be giving up by doing this?
      3) Is it possible to adjust the parameters, either manually or automatically, between the Sunny Island and the Sunny Boy to limit the current output of the SB7000US that will not overload the Smartformer? There is a comm cable running between them, and the SB already has current limiting capability built into it – what are we missing?
      4) If these questions are not posed with the correct operational conditions in mind, is there more precise documentation on what the correct operational conditions are? Along these lines, at a minimum we would like to ensure that when we’re in “grid up” mode, that the SB7000US is able to produce unconstrained power back to the grid – do we need to provide our own switching for this. Please help us to fill in some of this – much appreciated.

      Thank you for any assistance you can provide.

      Reply »
      • avatar

        Greg Smith

        02/12/2013 at 11:17 am

        Hi Scott,

        We are definitely delving into questions that are better suited for a phone conversation. Please call our Service Line at 877 697 6283.

        Greg

      • avatar

        Petar Aladrovic

        05/10/2013 at 12:33 am

        Hi Scot and Greg,

        I am trying to make the same solution with Sunny Island and few Sunny Boys. I want to power up the whole building during the day and partly during the night through the batteries.
        Question is only: I need to ensure also the grid connection during cloudy days. With current solution from Sunny Island, it seems to me that I will be also powering the grid (or the generator) when I have enough power from PV.
        For generator in isolated mode this would not be the problem, but with public grid it is a little bit tricky. I am sure that automatic switch must be added between public grid and Sunny Island ? Or ?

        Petar

      • avatar

        Melissa Womack

        05/15/2013 at 8:18 am

        Hi Petar,

        You must use an automatic transfer switch in a grid tied application with a generator. Our installation manual provides guidance – here is a link to the download section of our website: http://www.sma-america.com/en_US/services/downloads.html. If you need any help feel free to call us at 1-888-4SMAUSA.

        Sunny Regards,
        Melissa Womack

  • avatar

    Melissa Womack

    11/08/2012 at 3:02 pm

    Hi Stephen-

    Thanks for following our blog, http://www.SMAInverted.com! If you have any additional questions please feel free to contact our service department, they are happy to give you immediate assistance.

    http://www.sma-america.com/en_US/services/overview.html

    Melissa
    Social Media Specialist

    Reply »
  • avatar

    […] Coupling Options For Off-Grid Solar Arrays | SMA Inverted – Substantial improvements to off-grid photovoltaic technology during the past decade have led to more choices in off-grid PV system design…. […]

    Reply »
  • avatar

    BriAN

    12/18/2014 at 3:07 pm

    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 »
    • avatar

      Justin Dyke

      12/19/2014 at 8:52 am

      Hi Brian,

      For your system to become grid-tied with battery backup you’ll need to add a Sunny Island and Smartformer with the appropriate amount of battery storage you’d need to power your protected loads during a grid failure. This would most likely work with your existing generator and transfer switch. To better help, is your generator 120V or 240V? Also, how much time do you expect to be without grid power or how often does the grid go down currently?

      Best,
      Justin

      Reply »
      • avatar

        BriAN

        12/24/2014 at 2:26 pm

        Justin,
        I have a Generac GP6500E. It has Four 20 Amp 120V receptacles, one 30 Amp 120/240V locking plug receptacle.
        How much time do I expect to be w/o power? My goal is to be able to run essentials. Allow the batteries to be charged by panels and generator, if needed.

        Currently, we seldom loose power – maybe a couple times a year for a few hours.

        -=-
        Justin Dyke

        12/19/2014 at 8:52 am

        Hi Brian,

        For your system to become grid-tied with battery backup you’ll need to add a Sunny Island and Smartformer with the appropriate amount of battery storage you’d need to power your protected loads during a grid failure. This would most likely work with your existing generator and transfer switch. To better help, is your generator 120V or 240V? Also, how much time do you expect to be without grid power or how often does the grid go down currently?

        Best,
        Justin

      • avatar

        Justin Dyke

        01/05/2015 at 10:38 am

        Hi Brian,

        The best next step would be to contact your system installer (or distributor if you self-installed) to determine which batteries they recommend and go through the load calculations for the needed amount of amp hours you’d like to have ready. This is all fairly basic for most installers and distributors to help with, but if you have any further questions please don’t hesitate to ask.

        Thanks,
        Justin

  • avatar

    Manny Rentetia

    12/19/2014 at 6:48 pm

    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 »
    • avatar

      Justin Dyke

      01/06/2015 at 1:04 pm

      Hi Manny,

      I have forwarded your question to a member of our Applications Engineering team to follow up with you directly. They can help with load sizing for batteries and getting a system design mocked up for your customer.

      All the best,
      Justin

      Reply »
  • avatar

    […] Coupling Options For Off-Grid Solar Arrays | SMA Inverted – Substantial improvements to off-grid photovoltaic technology during the past decade have led to more choices in off-grid PV system design…. […]

    Reply »
  • avatar

    Steven Kong

    01/22/2015 at 1:39 pm

    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 »
    • avatar

      Justin Dyke

      01/26/2015 at 4:18 pm

      Hi Steven,

      Unfortunately the Sunny Tripower TL-US inverter is not compatible with the Multicluster Box in North America. The Tripowers are not equipped to handle the frequency shift and communication from the Sunny Islands. As of right now, the largest SMA North American inverters that will work with the Multicluster Box are the Sunny Boy 9/10/11000TL-US inverters.

      Best,
      Justin

      Reply »
  • avatar

    Carmen

    06/09/2015 at 9:24 am

    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 »
    • avatar

      Justin Dyke

      06/09/2015 at 11:17 am

      Hi Carmen – All of our Sunny Boy inverters need to have the AC grid to operate. The only way to power something without a grid interconnection is through our Sunny Island for off-grid systems. If that’s something you’re interested in learning more about, please let me know and I can send over some information.

      Best,
      Justin

      Reply »
      • avatar

        Carmen

        06/10/2015 at 1:08 am

        Thanks Justin.

        I’m asking this in light of academic knowledge. So the Secure Power Supply function can only be used from the battery supply? And the Sunny Island also needs a battery bank?

        I’m looking into a dedicated system, since the pump does not need to run if there is no sunlight and therefore a battery bank would unnecessarily increase the system costs.

      • avatar

        Justin Dyke

        06/10/2015 at 8:05 am

        Hi Carmen – Secure Power Supply can be used without the AC grid present, but it is limited to 1,500 watts, which I doubt will run a pool pump. For the Sunny Island, it would need a battery bank and you would still need the Sunny Boy to handle the incoming PV power from your modules.

        It sounds like the best option for you is a DC charge controller with a small battery bank to power the pump. That would likely be the most cost efficient and simple way to operate your pool pump.

        Also, have you looked into any of our Solar Academy training webinars? They are free and can teach you a lot more about each of these system designs than I can 🙂

  • avatar

    Gavin

    07/21/2015 at 2:47 am

    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 »
    • avatar

      Justin Dyke

      07/21/2015 at 11:16 am

      Hi Gavin,

      Yes, it’s entirely possible. A good diagram example is available here, but please note that it doesn’t include a shunt for the Sunny Islands, which we highly recommend using. I have a better diagram in PDF if you’d like, just email socialmedia@sma-america.com and I can send it over.

      Best,
      Justin

      Reply »
  • avatar

    Harsh

    10/27/2015 at 7:15 am

    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 »
    • avatar

      Justin Dyke

      10/27/2015 at 4:40 pm

      Hi Harsh,

      This all depends on the types of loads and time of day you’d need to power them. Unfortunately it’s a much more complicated answer. Feel free to email more system details to SocialMedia@SMA-America.com and I can connect you with someone to discuss DC versus AC coupling further.

      Reply »
  • avatar

    john

    03/23/2016 at 8:13 am

    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 »
    • avatar

      Justin Dyke

      03/23/2016 at 4:29 pm

      Hi John,

      We are not familiar with the non-North American systems can thus can’t comment on proper battery sizing or system design questions. Have you tried talking with someone in Australia? I can connect you with their team if you haven’t contacted them yet.

      Best,
      Justin

      Reply »
  • avatar

    Doug

    05/18/2016 at 11:11 am

    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 »
    • avatar

      Justin Dyke

      05/19/2016 at 2:30 pm

      Hi Doug –

      Those are great questions. I have forwarded them to our applications engineering department and a technical expert will contact you soon.

      Best,
      Justin

      Reply »
  • avatar

    Stephen More

    05/31/2016 at 6:47 am

    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 »
    • avatar

      Justin Dyke

      06/09/2016 at 3:47 pm

      Hi Stephen –

      The currently available Sunny Boy Storage, as it is based on a PV inverter not certified for U.S. use, will not be sold in the U.S. A UL-listed device that will be similar, but not identical, is on the product roadmap and will be available in the US at a later date, likely in 2017.

      No specifications have been released for the UL-listed version of the Sunny Boy Storage. However, the current Sunny Boy Storage is a current source or sink, and is not able to form a grid should the utility fail. Without a device like the Sunny Island to safely create and manage a micro-grid when the utility fails, both the Sunny Boy and Sunny Boy Storage would stop exporting and importing power in the case of a grid failure.

      Best,
      Justin

      Reply »
  • avatar

    Tim Fitz

    06/25/2016 at 11:37 am

    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 »
    • avatar

      Justin Dyke

      06/28/2016 at 6:57 pm

      Hi Tim –

      Thank you for your questions. I have shared your information with the applications engineering team and a technical expert will be in contact soon.

      Best,
      Justin

      Reply »
  • avatar

    linda verwilligen

    08/05/2016 at 8:29 am

    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 »
    • avatar

      Justin Dyke

      08/05/2016 at 1:41 pm

      Hi Linda –

      Those are great questions. I have forwarded your information to the applications engineering department and a technical expert should be in contact soon.

      Best,
      Justin

      Reply »
      • avatar

        gary taylor

        08/28/2016 at 9:29 pm

        The sunny boy island inverter/charger schematic indicates the batteries and Pv panels are in parallel and not in series. DC coupled is PV into charger controller into batteries into sunny boy island. The schematic also indicates the sunny boy is a stand alone master inverter or may acts as a slave inverter in parallel. I want to connect the sunny boy island inverter/charger up as a master inverter that regulates supply from PV or Grid or batteries to the house load as a stand alone inverter. Does the PV go into the sunny boy island inverter/chager therby eliminating the need to a PV charge controller? Thanks Gary

      • avatar

        Justin Dyke

        09/02/2016 at 5:22 pm

        Hi Gary –

        That is a great question for one of our technical experts. I have shared your information with the Service Line department and a team member should be in contact soon.

        Best,
        Justin

  • avatar

    Danny

    09/11/2016 at 6:45 pm

    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 »
    • avatar

      Justin Dyke

      09/21/2016 at 11:42 am

      Hello Danny,
      Thank you for your question. Yes, you can use a Sunny Tripower TL-US instead of three Sunny Boy inverters, but you will need to use a transformer for AC Coupling. Keep in mind, the Sunny Island output voltage is 120 VAC and the Sunny Tripower’s output is 480 VAC.

      Reply »
  • avatar

    Thorbjorn Waagstein

    11/16/2016 at 7:42 am

    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 »
    • avatar

      Jessica

      11/17/2016 at 10:23 am

      Hi Wes,

      The Sunny Island can accept a 120vac generator input. If you have a 220vac generator and only one Sunny Island, you will have to ask your generator manufacturer if the generator can handle having only 1 leg (L-N) loaded down. If you have two Sunny Islands, each unit would connect to a phase of the generator (L-N) at the AC2 terminal.

      From your comment, it sounds like there is a Utility Grid as well. If that is the case the Sunny Island will need the Digital Input Circuit installed so that the Sunny Island knows whether to load Grid or Generator parameters.

      The Generator cannot connect to AC1 (critical loads panel), it must connect to AC2 (Grid/Generator Input).

      If you need additional info please reach out to our service line at service@sma-america.com or 1 877-697-6283

      Reply »
  • avatar

    Wes

    11/16/2016 at 9:57 am

    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 »
    • avatar

      Jessica

      11/16/2016 at 3:49 pm

      Hi Wes,

      A member of our service team should be reaching out soon to answer your question.

      Thank you!

      Reply »

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