Advantages of AC-coupled high-voltage-battery over alternative solutions?

High voltage batteries only arrived in the market in the last 12 months. However their arrival has allowed a number of other technologies to changes the way battery storage can be included in the home. AC-coupled storage using these new technology high voltage batteries (i.e. > 120V DC) has a number of advantages for homes wanting to include battery storage with their solar system. In addition to being a lower cost solution compared to alternatives, AC-coupled storage with high voltage batteries can allow your home to increase its independence from the grid and provide you with greater flexibility in the future to change your system to meet the needs of a growing or changing home. AC-coupled storage with high voltage batteries is also typically a more efficient system meaning you’ll get more energy and power compared to a similar hybrid inverter system. And perhaps best of all, AC-coupled storage can turn any new or existing solar system into a true battery-ready system.

Read on to discover the benefits of AC-coupled storage with high voltage batteries and why it can be the best solution to meet the individual energy storage needs of your home.


What is AC-Coupled Storage?

AC coupled storage is the connection of a battery energy storage system to a solar system via AC (alternating current) electricity. Energy from a solar system is generated in the form of DC (direct current) electricity which is then turned into AC by the solar inverter. A battery inverter connected to the same electrical network via AC then converts any spare energy not used by the home back into DC so it can be stored in a battery. By contrast DC coupled storage uses what is commonly referred to as a hybrid inverter, where both the solar and battery are connected to the same inverter. However, as we’ll see below there are many reasons why AC-coupled storage using high voltage (HV) batteries offers a number of advantages in terms of cost, flexibility and risk compared to DC coupling.




Turn ANY PV System into a “Battery-Ready” System

bild-3-any-pvOne of biggest advantages of AC-coupled storage is that it turns ANY new or existing solar system, into a true “battery ready” system. Since batteries and solar cannot be simply connected together, grid-connected battery storage will always be required to connect to an inverter. AC coupled storage means that the inverter to which the battery is connected is separate to the solar inverter. It can be a much simpler installation in many cases by using two separate inverters for battery and solar Even though there are 2 separate inverters, compared to a number of hybrid inverter solutions it can still represent few components overall. The other main advantage is that for existing solar systems, the investment already made does not need to be wasted by removing the system to install a hybrid inverter. With no more than 3 components (shown inside the red dotted line) a new or existing solar system can be “battery ready” thanks to AC-coupling.


Lower Cost Solution

AC-coupled storage with HV batteries offers opportunities for lower cost products and installations As recently as early 2016, HV batteries like those from manufacturers such as LG Chem or BYD were not available in the market. With their introduction however, lower costs are able to be realised with respect to the inverter. Since a HV battery is similar in current and voltage characteristics to a normal solar array, they can utilise similar transformerless inverter technology. In addition to being more efficient than traditional transformer-based inverters, transformerless inverters are also significantly cheaper since they are much smaller and use far fewer components (this also can make them more reliable!). This means that you can add battery storage to your new or existing solar system more cheaply.

One of the realities of a technology industry like solar is each year, the technology gets better and cheaper. So if you are installing a new solar system but want to wait a few years to install batteries, AC-coupling with HV batteries will allow you to do this at a lower cost in the future compared to choosing a DC coupled hybrid inverter now and buying batteries later. This is because in the years between installing your battery ready PV system and buying your batteries, technology will have got cheaper for both the inverter and the battery. It can also be worth considering when installing a solar system, whether you should install a battery or wait some time to determine your self-consumption and then decide on the correct sized battery for your individual needs. If however you originally decided to install a hybrid inverter, you realised a higher cost for the technology rather than being able to purchase AC-coupled storage at a lower future cost. The other advantage about choosing AC-coupled storage rather than a DC coupled hybrid inverter is that you still have the opportunity to take advantage of future technology developments. For example, if you had invested in a hybrid inverter in 2014 and decided to buy batteries in a few years, you would not have been able to take advantage of newer and lower cost battery solutions like those from LG Chem and BYD. With AC-coupled storage, you can take advantage of such future developments because the solar and battery systems are independent of each other.

Whether it is a transformerless inverter, future reductions in the cost of products or as-yet undiscovered technology advancements in batteries, AC-coupled storage offers a lower risk and lower cost option for adding battery storage to your solar system.


Flexibility of Installation Location


AC-coupled storage provides a greater degree of flexibility, and potentially at lower cost. When adding batteries to a new or existing solar system, one main advantage of AC-coupled storage can be the flexibility of where the battery system is installed. Many solar systems have the inverter installed either on an external wall or perhaps in a sealed services room. These might not be the best or even suitable locations for the installation of the battery system. AC-coupled storage means the battery system can be installed wherever it is best to be installed (e.g. hidden in a garage, etc), and independent of the solar system. This can also be helpful for keeping the cost of installation to a minimum. Different factors affecting each part of the system and its installation, can be addressed independently and potentially avoid additional work required to make an install location suitable for both solar and batteries.


Flexibility regarding Upgrades or Changes to the System

Homes change over time. New children arrive or they may finish school and either study at university or move out. Maybe you reach retirement and are spending more time at home during the day. Or perhaps you change your home appliances, buy an electric car or add new home appliance that are still to be invented (after all HV batteries for the home didn’t exist only a few years ago!). As homes change, so too does the consumption of energy. This means energy storage (batteries) and energy generation (solar) systems may also need to change. AC-coupled storage means both battery and solar systems can be independently modified without necessarily affecting each. If a home starts to use more energy during the day, the solar system can be upgraded or changed to meet this energy demand without touching the AC-coupled storage. Similarly, if a home starts to use more energy during the evening when electricity prices tend to be higher, the battery system can be upgraded to meet this demand with no changes to the solar system.

AC-coupled storage provides this flexibility to change battery or solar independently which means future potential costs will be lower compared to a similarly sized hybrid inverter solution. This is also important to remember as technology and standards change which, for hybrid inverter systems, may require the both parts of the system be brought up to code if one of them is modified. This can add significantly to modification costs. With AC-coupled storage, system upgrades or changes are more flexible and can be realised at lower future cost and risk.


Increased Independence from the Grid

When you add a battery system to your solar system, the main purpose is to better self-consume the energy from your solar system and increase independence from the grid. AC-coupled storage can allow you to have maximum grid independence compared to a hybrid inverter solution since you are able to supply loads in your home from both the solar and battery simultaneously. The example below shows what would happen for a typical single phase home which has solar and storage. Restrictions in the allowed capacity to connect to a network would mean that an AC-coupled system could be supplying 100% of your home’s load requirements compared to a DC coupled hybrid inverter which might only supply 66% of your home’s load. The flexibility of AC-coupling means that you will increase you independence from the grid and in doing so, reduce your costs related to purchasing grid electricity.




Simplified and Improved Blackout Operation


Although modern power grids rarely experience blackouts, one of the drivers you may have when purchasing a battery system is to “blackout-proof” your home (also referred to as backup). This would mean installing and setting up your system so it is able to disconnect from the grid and continue operating. Especially for existing solar systems, AC-coupled storage offers a greatly simplified means of achieving backup. For additional cost and by adding some simple changeover switches which then interface to the AC-coupled battery inverter, any solar system can also provide backup power. And if the solar inverter is able to have a Power-Frequency response programmed into it (SMA or 3rd party solar inverter), the solar system will also be able to add power to your “blackout-proof” system. If this were compared to a system where a hybrid inverter with additional solar array were added to an existing system, backup power would only be provided from the hybrid inverter and not the other existing Solar system. This type of system simplifies how backup is achieved so that, in the unlikely event of a grid failure, your system can provide power to keep your lights on and your fridge cold.


More Energy and Power from Better Operating Efficiency

One of the best advantages which has come from the development of HV battery technology for home energy storage is the ability to connect them to higher efficiency transformerless inverters. Low Voltage batteries need to connect to transformer-based inverters which, apart from being much heavier, are less efficient when they convert between DC and AC. Most hybrid inverters available on the market are designed to connect to Low Voltage batteries which mean they use a less-efficient transformer as part of their system. Even though AC-coupled storage may have more energy conversion steps in the process, since the HV batteries are connected using more efficient transformerless inverters, the energy delivered from the battery to the loads in your home can be at a higher efficiency. This means that your AC-coupled storage system with HV batteries will deliver more energy and power compared to a similar hybrid inverter system. The image below shows what the theoretical maximum conversion efficiencies are for both AC-coupled storage with HV batteries and DC coupled storage with LV batteries.




Both systems seem reasonably similar, however this does not then take into account the additional losses from the battery. The curve below shows the round trip efficiency between Solar ⇨ Battery ⇨ Home loads.



This shows a far greater difference in total system efficiency meaning AC-coupled storage with HV batteries could potentially deliver far more energy (hundreds of kWh) per year compared to the DC coupled hybrid inverter system. For a typical family home, this increased efficiency allows greater grid independence which could help you avoid hundreds of $ / € / ₤ of grid energy supply charges per year.



AC-coupled storage can turn any new or existing solar system into a battery-ready system unlike alternate DC coupled / hybrid inverter solutions. With the introduction of new high voltage batteries, AC-coupled storage has become a lower cost option to add battery storage to a solar system compared to hybrid inverters or low voltage battery storage. AC-coupling also offers a number of advantages such as flexibility for installation and also future upgrades or changes to either the solar or storage system. This means the system is better able to meet the individual needs of your home and can allow you to add battery storage at a lower cost. AC-coupled storage also allows you to better increase your independence from the grid saving further on electricity supply charges. This financial benefit is also increased through a more efficient operation. And with a simplified means to blackout-proof your home, AC-coupled storage with high voltage batteries is the smart solution if you want to add battery storage to your solar system.

4.2/5 - (6 votes)
  1. Wayne
    Wayne says:

    2x Sunny Islands is out of my price range. Are there any Split Phase inverters, like Magnum, that will work correctly with AC Coupling to an SMA SB7.0-1 SP-US-41? Any hybrid inverters tested? Thanks!

  2. Helen Rutherford
    Helen Rutherford says:

    HI we have a 10kva grid tie system 2009 install with 2xSMA sunny mini central 6000A Can this be ac coupled using sunny island or similar to povide battery and outage backup
    we are in FNQ AU

  3. solar student
    solar student says:

    thank god there are SMA made explanations on the internet. I have been looking high and low until i finally stumbled on the correct designation “AC-coupling”!
    now I can just mention or search this term and people understand!
    “Charging Battery from the battery inverter AC-OUT side” just gave me perplexed looks. This needs to be advertised or mentioned more.
    People new to solar “stuff” have a hard time as it is …

  4. cris
    cris says:

    i. Good day! Is the current stored in battery can used automatically @ nigth when the solar is not worker? or it can only use during block out? Thank you

  5. Moh
    Moh says:

    It is difficult to believe that the efficiency of charging battery from the solar panels in the case of AC coupling where two conversion stages are used is less than hybrid case where only one conversion stage is required.

  6. Uttam Chauhan
    Uttam Chauhan says:


    Please provide SMA product reference for AC Distribution Panel. Is it SMA SI-TD-BOX-10 SI TRANSFORMER. Will it take care of not back feeding to the Grid when grid is off (blackout) which is utility requirements.

    Or it will need more product to achieve this goal.



    • Anke Baars
      Anke Baars says:

      Hi Uttam,
      Please note: The Smartformer Box for Sunny Island SI-TD-Box-10 has been discontinued since the beginning of 2017 and is no longer available. If you still have one at your disposal, I can send you the corresponding documents by mail. Just give me a note. Or please provide details about your configuration and we´ll see how to help you with the design.
      Kind regards,

  7. Alf Carubia
    Alf Carubia says:

    Hi im accredited solar installer and have done my battery course but not accredited through cec for battery installs
    Could i team up with someone in adelaide who uses this AC coupled battery system so i can eventually get my battery cec battery endorsement

  8. Victor
    Victor says:

    it is done that nothing is done 🙂
    If a three-phase grid tie inverter is already in operation (for exemple STP9000), then a hybrid inverter must be used in parallel, which will provide power in the event of a blackout.

  9. Alex
    Alex says:

    Hi, I realise this is an old thread but you seem to be answering questions still so here’s my slightly unusual situation… I have an Enphase system with micro inverters so essentially get ac supply from the panels. I also have an LG RESU10H battery which I would rather connect to my system than sell. I’m not sure I totally understand your article so rather than making any assumptions, what suggestions do you have for how to incorporate the battery into my energy management?

  10. Nosa
    Nosa says:

    Hello SMA,
    I have gone through your AC coupled system and the advantages stated. But i have one question, the AC coupled system you showed here have Grid input, in the case where their is not Grid or the Grid is not always constant, what then can you use as Grid for your PV Inverter to mirror before it can start supply, assuming the batteries are totally drained.

    • Scott Partlin
      Scott Partlin says:

      Thanks for reading and for your question.

      SMA’s most recent release of AC coupled battery inverters (Sunny Boy Storage 3.7/5.0/6.0) is able to operate during a grid outage either with the built-in Secure/Emergency Power Supply, or via the optional external Automatic Backup Controller. So when the grid is present, it can operate to reduce energy consumption from the grid and store excess solar energy. But when the grid disappears during an outage it is able to generate it’s own grid to power household loads and (if using the Automatic Backup Controller) allow the Solar inverter to also work (Single phase inverters only).

      In a case where, when operating in a backup mode during a grid outage, and the batteries became completely drained, it would have to wait for grid power to return for the unit to start again and allow the batteries to be charged from either Solar or grid. But for most common grid connected storage applications, a grid outage longer than 24 hrs is extremely rare so the likelihood of such a situation arising is similarly extremely low.

      • gse-bf
        gse-bf says:

        As the Sunny Boy Storage 3.7/5.0/6.0 is working on single phase system basis, is it possible to use 3 pieces of them to build a three-phase system power source?

      • Anke Baars
        Anke Baars says:

        Thanks for your request.
        It is only possible to configure single-phase systems with Sunny Boy Storage (SBS). In a three-phase infrastructure, however, the SBS + battery bank can be connected to any of the phases.
        Kind regards,

  11. Neil
    Neil says:

    Hi SMA,

    we have a sbs 2.5 and an lg chem resu 10H. Charging is working fine but how can we limit the depth of Dicharge. Battery percentage always goes down to zero. hope you can help us. thanks

    • Carolyn Schlosser
      Carolyn Schlosser says:

      Hi Neil,

      Please use the settings in the following:
      -consumption of auxiliaries
      -battery discharge limit (the lower part)

      Now you can choose to which percentage your battery can discharge.

      Sunny regards,

  12. Derek Kroes
    Derek Kroes says:

    I need a 12kW split phase battery inverter. 360V dc input. Several systems. Could you have someone call me?


  13. James Curtis
    James Curtis says:

    Very good read. With the efficiency part what was the other inverter you were comparing to with the D.C. Battery? Would the results be the same with the battery ready Solar edge transformer inverters?

    • Scott Partlin
      Scott Partlin says:

      James, it would be a bit unprofessional I think to answer that question on a public forum. Suffice to say though, there is no reason to think that a solution from a different manufacturer would be much different. The main issue is the topology and configuration. Manufacturer differences can only slightly optimize constraints based on physics

  14. Luther McFarlane
    Luther McFarlane says:

    It seems as SMA has not paid much attention to North America. I am using the SMA6000TL-US which is a split phase 240V inverter. However your product to add AC coupling are all 240V 1ph or 120V 1ph. Now to make ac coupling work you would have to use a autotransformer which adds inefficiency to the system. The only other option is to use two Sunny Island inverters which is quite expensive. When will SMA take the US & Canadian market serious enough to start making attractive add on for the Grid Tie inverters that it has sold over the years. Right now Schneider, Outback and a few others have made low voltage battery inverters to supplement AC coupling with your SMA grid tie inverter. The high voltage AC coupling you described in this article is quite interesting but not applicable to North America which uses a split phase 1ph 240V system. When will you start providing efficient retrofit to AC couple your currently installed grid tie inverters.


    Thanks for the article.I have some questions about back-up and whether the sunnyboy storage can be stacked. I am looking for a retrofit solution to 10 kw of solar via 2 x 5kw string inverters ( non sma).
    Is there any more detail about how the changeover switch is integrated

    • Rhea Terlinden
      Rhea Terlinden says:

      Hello Robert,
      at this stage, the Sunny Boy Storage is only able to be installed as a 1 unit per installation site. Multiple units (Phase stacked or on different Phases) is not possible. It may be a capability which is added in future but there are no plans currently to realise this.
      Best regards

  16. chris Packer
    chris Packer says:

    I’d be interested to find out how this could work with 3-phase inverters such as stp17000tl-20?

    • Jennifer Rößler
      Jennifer Rößler says:

      Hello Chris,
      For residential customers, it does not matter if the battery storage is single or 3-phase, since customers are billed on a nett basis rather than an individual basis. Given this, adding a single phase Sunny Boy Storage to a 3-phase PV system will give the owner the same financial return as if the battery inverter were 3-phase.
      If you want to install 3 battery inverters, one on each phase, at the moment this is not possible with Sunny Boy Storage, only with Sunny Island since that platform has the necessary master-slave configuration between inverters.
      Kind regards, Jennifer

  17. Neranjan Rathnayake
    Neranjan Rathnayake says:

    This is brief, yet comprehensive explanation. With the capability of integrating into existing systems, AC system coupling will be more effective rather than hybrid inverter topology.

  18. Ralph Berg
    Ralph Berg says:

    Yes thanks for the informative article Scott, it makes a compelling case for AC coupled, especially considering the efficiency figures provided, which has been one of the arguments against AC coupling by the DC Coupling vendors. Looking forward to the higher capacity SBS units becoming available in the Australian marketplace.

  19. Anthony chijioke
    Anthony chijioke says:

    Thanks for the information. A good insight to my new interest in solar power generating project. Would like to work out something with you sometime.

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