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

One 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.

Conclusion

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.