ac coupled vs dc coupled batteries

AC vs. DC solar battery coupling: what you need to know

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Solar batteries are becoming popular additions to solar energy projects of all scales. When it comes to the way your solar panels, batteries, and inverters are all wired together on your property, there are two main options: alternating current (AC) coupling and direct current (DC) coupling. Both AC and DC coupling have advantages and drawbacks that are dependent on the specifics of your solar plus storage installation.

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AC vs. DC coupling: what’s the difference?

The key distinction between an AC-coupled and DC-coupled system lies in the path electricity takes once it is produced by solar panels. Solar panels generate DC electricity that must be transformed into AC electricity for your home’s appliances. However, solar batteries store electricity in DC form. 

In an AC-coupled system, DC solar electricity flows from solar panels to a solar inverter that transforms the electricity into AC electricity. That AC electricity can then flow to your home appliances, or go to another inverter that transforms the electricity back to DC for storing in a battery system. With AC-coupled systems, any electricity that is stored in the battery system needs to be inverted three separate times before use.

ac coupled battery system diagram

In a DC-coupled system, DC solar electricity flows from solar panels to a charge controller that directly feeds into a battery system, meaning there is no inversion of solar electricity from DC to AC and back again before the electricity is stored in the battery. Any electricity produced by the solar panels will be inverted only once (from DC to AC), either as it flows from batteries to your home electrical setup or out to the electrical grid.

dc coupled battery system diagram

Historically, AC-coupled battery storage setups have been more common for residential and commercial solar installations, but as more DC options become available, DC coupling is gaining in popularity.

Pros and cons of AC coupling

The main advantage of AC-coupled solar battery systems is their ease of installation, especially for retrofit storage installations. Easier installations require less labor and time for solar installers, which often means a lower upfront cost.

An additional benefit of AC-coupled systems is that these setups allow batteries to charge from both solar panels and the grid. This means if your solar panel system isn’t generating enough electricity to fully charge your battery, you can still rely on the grid to fuel your battery for resiliency or electricity rate arbitrage benefits. This is also an important advantage if you’re hoping to participate in a utility energy storage pilot or another type of demand response program, as your electric utility may need to be able to control the flow of electricity in and out of the battery. 

As far as drawbacks are concerned, AC coupling means that stored solar electricity will need to be inverted three separate times before being used by home appliances. The process of inverting electricity from AC to DC, or from DC to AC, results in small efficiency losses, so the more inversions that take place, the larger the overall reduction in system efficiency. Most inverters have efficiency losses of about a few percentage points.

Pros and cons of DC coupling

DC-coupled solar energy systems have the advantage of being more efficient than AC-coupled systems. While solar electricity is converted between AC and DC three separate times in an AC-coupled storage setup, DC systems convert electricity from solar panels only once, leading to overall higher system efficiency.

That being said, DC-coupled options are more complicated to install, which can drive up upfront costs and installation time.

Which solar plus storage system is best for you?

If you already have a solar panel system installed on your property and are looking to add battery storage as a retrofit, an AC-coupled system is likely best for you. This is because you’ll already have a solar inverter system installed with your panels, and rewiring for a DC-coupled system is a complicated process that can increase installation costs.

That said, if you’re installing a solar panel system and battery setup at the same time, a DC-coupled system may be the better option because of the higher overall efficiency of DC-coupled setups. However, the installation time for DC-coupled systems is usually longer than for AC systems, so it’s important to factor in how that will impact your upfront installation costs.

See if solar plus storage is right for you

Regardless of whether you choose an AC or DC coupled system, installing a solar plus storage system on your property can be a great way to save money. Check out EnergySage’s Solar Calculator to see how much solar alone can save you, and register for the EnergySage Marketplace to receive quotes for solar (and solar plus storage) from local solar companies licensed to install these systems.

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6 thoughts on “AC vs. DC solar battery coupling: what you need to know

  1. Maximo Torres

    This is a good article but not necessarily accurate. Technology is moving fast into transformer-less DC/AC hybrid inverters with integrated Mppts chargers and connection box. This takes away the complexity of independent components. Also All in one units that incorporate batteries are way easier to install as everything comes and go to the ESS system. I work with both AC and DC coupled. AC coupled will likely be preferred in old installations and DC coupling in new installations. However, sometimes the customer has a system older than 5 years with lower conversion efficiency and installation the ESS in DC coupled will likely increase its conversion efficiency. This has to do mainly to old transformer type architecture vs new transformers-less architecture already implemented years back in grid tie inverters. An addition benefit is longevity. Ac couple is an on/off switch, dc coupled is a voltage regulated pulsating process with improve charging process.

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  2. Brett Henning

    re: DC-coupled vs AC-coupled efficiency – In a DC-coupled system the charge controller, or DC-DC converter, that processes power in and out of the battery system is a conversion step with efficiency losses that are roughly the same as modern solar (PV) and storage inverters (with 98-98.5% rated efficiencies). So for a DC-coupled system the number of conversion steps is actually the same as in an AC-coupled system: [charging] from the PV array through the DC-DC converter to the battery (1), [discharging] from the battery through the DC-DC converter to the PV inverter DC input bus (2), and through the PV inverter to the load or grid (3) – three, same as for AC-coupled systems. So when charging from solar the overall throughput efficiency is essentially the same for either system topology.

    When charging from the grid, however, an AC-coupled system is actually much more efficient. Follow the electron and count the conversion steps: AC-coupled – [charging] from the grid through the storage inverter to the battery (1), [discharging] from the battery back through the storage inverter to the load or grid (2); DC-coupled – [charging] from the grid through the PV inverter* (1), from the PV inverter DC bus through the DC-DC converter to the battery (2), [discharging] from the battery through the DC-DC converter to the PV inverter DC bus (3), through the PV inverter to the load or grid (4). That’s two conversion steps for an AC-coupled system and four for a DC-coupled system. In applications where charging from the grid is feasible and beneficial (or may become so within the lifetime of the system), efficiency is arguably more important than when charging from solar – since grid power will come at some cost while solar power is essentially free.

    I am not certain, but the notion that DC-coupled systems are more efficient may come from grid scale PV+Storage systems where large central PV and storage inverters are each integrated with dedicated step-up/isolation transformers. The additional transformer losses going between the PV and storage inverters would definitely make an AC-coupled grid scale PV+Storage systems less efficient (though there are other factors that may favor AC-coupled systems in some grid scale applications). In residential, commercial, and other distributed generation applications, modern transformerless grid-interactive PV and storage inverters can be AC-coupled on the same low voltage AC bus without any intervening transformers.

    * In a DC-coupled system charging from the grid requires a bi-directional PV inverter which may be more expensive than typical PV inverters.

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  3. Scott Wynhoff

    One thing not mentioned in your article is that DC-coupled systems can be charged either by solar or the grid. Specifically The Generac PWRCell. This system also has the ability to be charged by a Generator. One other overlooked item is that an AC-coupled system will shut down the solar panels during the sunlight hours if the grid goes down. The problem is, it has nowhere to go with the excess power when the grid is down especially if the batteries are already charged. It is not until the batteries need charging that the panels will reactivate. Just an FYI

    Reply
    1. Jun Y Li

      But isn’t it the same thing for the DC-coupled system?
      When the grid is down, the solar won’t charge the battery when it’s full. Where does the excess solar energy go in that situation?

      Reply
      1. Nick B.

        When the battery is full and the grid is down, the excess energy produced by the DC coupled PV array would power the loads running on the critical loads panel or on the main panel if using the Generac ATS for whole home back up.

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