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AC vs. DC Coupled Batteries: Which One Is Right For You?

Kyle Ritland

May 2021

powerwall & generac-1Energy storage has grown increasingly popular in recent years, and with good reason. Batteries represent the next step toward grid independence, effectively expanding the capabilities of your solar panels around the clock by allowing you to store and use your excess solar energy, both day and night.

With interest among homeowners and businesses growing, the battery backup systems available for consumers have grown as well. Here at Sun Valley Solar Solutions, we currently offer both the Tesla Powerwall and the Generac PWRcell, giving you a choice of DC-coupled or AC-coupled options.

So which one’s right for you? Let’s take a look at the difference between these two battery types.


AC-coupled vs. DC-coupled - What’s the Difference?

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

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

In most DC-coupled systems, DC power flows from your solar panels directly into your battery system for storage. Power from the batteries must still be transformed to AC power for use in your home, but that power is only inverted once since it flows directly from the panels into the battery through the charger controller.

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.


AC Coupled: Best for Those Who Already Have Solar

For homeowners who already have solar and are looking to add battery storage to their existing array, an AC coupled battery is generally the best solution, both in terms of cost and ease of installation. They commonly consist of lithium battery modules, a battery management system, and an inverter/charger, all in a single compact unit that can be easily installed and connected in most homes.

One benefit of AC coupled systems is that these setups enable batteries to charge from both solar panels and the grid. This means if your panels aren’t generating enough electricity to fully charge your battery, you can still rely on the grid to charge your battery.

One of the drawbacks of AC coupled systems is slightly lower efficiency. Inverting electricity from DC to AC or AC to DC results in small efficiency losses, and multiple inversions result in efficiency losses of a few percentage points.

tesla installer_general-1The AC coupled Tesla Powerwall (capacity 13.5 kWh) is a great option for upgrading your existing solar energy system since it charges directly from your solar panels, isn’t reliant on the power grid, and can provide power backup even when not fully charged. However, it’s important to note that a whole home backup system will require multiple batteries, and because the Powerwall charges directly from your solar array, you will need to ensure that you have enough panels to fully charge your battery while the sun is out.

With the Powerwall, you use the Tesla app to schedule the battery’s charging and discharging schedule. If your utility changes on-peak and off-peak times depending on the season, you will have to remember to update those times yourself when the time comes.


DC Coupled: Best for Those Installing Solar and Storage Together

If you’re planning your system from scratch, installing a solar array and battery at the same time offers several advantages. In this case, DC-coupled batteries are generally a better choice, offering slightly higher efficiency overall and fewer components to be hung on your wall. 

powerwall & generac

DC coupled systems have been used for decades in off-grid solar installations and smaller capacity systems like RVs and boats. The most common DC-coupled systems use solar regulators to charge the batteries directly from your solar array, along with an inverter to supply AC power to household appliances.

The Generac PWRcell (minimum capacity 9 kWh, expandable to 18 kWh) is a great choice for DC-coupled systems. It offers easy, streamlined installation, and is programmed by your installer to fit with your utility's schedule. You won't have to worry about programming it yourself or updating schedules depending on the season, and the PWRcell delivers precise demand control.

The PWRcell can be set up to charge off the grid or via your solar panels, making it a more viable option for smaller systems that may not provide enough power to fully charge the batteries. In addition, DC-coupled systems offer slightly higher efficiency due to the fact that there are fewer conversion gateways (AC/DC, DC/AC) in a DC-coupled system.  


Either Way, You Win

Whether you’re looking to expand the capabilities of your existing solar array, or just looking into installing a new system on your home, adding energy storage will dramatically up your game and result in significant savings and security for you. When your solar panels produce more electricity than you can use in your home, you can store that excess electricity instead of selling it back to the utility. This is a particularly important consideration now that utilities are largely reducing the credits they give you for excess solar power.

 

Energy storage systems are a smart investment in your system, and Sun Valley Solar Solutions can put together the perfect storage system for you. To learn more about energy storage options for your home, request a free quote today.

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