AC vs DC-Coupled Battery Storage: What You Need to Know

As the demand for solar power continues to grow, more homeowners and businesses are turning to solar power to reduce their electricity bills and decrease their carbon footprint. One important consideration when going solar is choosing a battery system to store excess energy generated by solar panels. Once the decision to install home battery storage has been made, the first question tends to be: should I choose AC or DC-coupled systems?

While these terms may sound unfamiliar, it’s important to understand the differences between the two to decide which is most suited for you. Luckily, they’re fairly straightforward concepts to wrap your head around. In this article, we’ll explore how AC and DC-coupled batteries work, the pros and cons of each system type, and how to choose which is best for your energy goals and setup. 

Key takeaways:

• AC coupling involves three conversions, resulting in minor energy losses that make it slightly less efficient. 
• DC coupling only involves one conversion that maximizes energy use for greater efficiency but DC coupled batteries can be more difficult to integrate into existing solar energy systems.
• If you want to add a solar battery to an existing solar installation, AC-coupled batteries may be better for you.
• If you’re building a new home or installing a new solar system, DC-coupled systems may be more suitable depending on your priorities.

What’s the difference between AC and DC-coupled systems?

Solar panels generate direct current (DC) electricity. This energy is then transformed into alternating current (AC) electricity, which is what's used to power most of your home's appliances. Solar batteries store electricity in DC form. To put it simply, the difference between AC-coupled and DC-coupled battery systems is whether the electricity generated by your solar panels is transformed before or after being stored in the battery. 

In AC-coupled systems, solar panels are connected to a solar inverter that transforms the DC power generated by the panels into AC electricity. This AC power can then be used by your home or flow to a battery inverter that converts the electricity back to DC power for storage. Electricity stored in the battery is then re-converted to AC before being used by the appliances in your home. In this way, AC-coupled battery systems require three inversions, which can result in some efficiency loss. However, AC-coupled systems  tend to be easier to install and are more compatible with existing solar panel setups. 

With DC-coupled systems, solar energy produced by your panels flows directly into the battery as DC power and only converted to AC when it needs to be used. This means there’s just a single inversion and less efficiency loss. DC-coupled systems are ideal for new solar installations as they’re much easier to install at the same time as the solar panels, rather than as an add-on to an existing solar array.. 

This might seem complicated at first, but understanding how AC and DC-coupled battery storage works is not as tricky as you think. Let’s take a closer look at how each process works and the pros and cons of AC vs DC coupled systems. 

See: How Does Solar Power Work?

What is AC coupling?

AC coupling is a method used to connect solar panels to a battery storage system. Alternating current (AC) is the type of electricity used to power your home. The large majority of household appliances use AC electricity. 

Solar panels, however, generate power in direct current (DC) form. This is also how batteries store electricity. Not just solar batteries, either - even your iPhone needs to convert AC to DC when charging. 

In an AC-coupled system, a solar inverter converts the DC electricity generated by solar panels into AC. This AC power can be immediately used for household appliances or fed back into the grid. To store this energy, a battery inverter must convert  the AC power back to DC. When it’s time to use the stored energy, the battery inverter re-converts the stored DC power back to AC. 

All up, an AC-coupled battery involves three stages of conversion: 

• DC power from solar panels to AC by solar inverter
• AC power from solar inverter to DC for battery storage
• DC power from solar battery to AC for home use. 

A small amount of power is lost at each stage of the conversion process, making AC-coupled batteries less efficient than their DC counterparts. Generally, AC-coupled systems tend to have an efficiency of 90-94% compared to 98% for DC systems. 

Examples of AC-coupled battery systems include the Tesla Powerwall, Enphase AC, or SonnenBatterie.

Pros of AC-coupled batteries

Let’s look at some of the benefits of using AC-coupled systems. 

Easy retrofitting

One of the main advantages of AC-coupled solar battery systems is their ease of installation when retrofitting storage to an existing solar system. If you already have solar panels installed, adding AC-coupled battery storage can be relatively straightforward. They’re also a lot easier to integrate with your home’s existing circuit panel and wiring. Easier installations require less labor and time for solar installers, which often means lower installation costs.

Backup power during outages

AC-coupled solar batteries provide backup power during grid outages. During a power outage, grid-tied solar systems will shut down unless you have battery backup.  With AC-coupled battery storage, you can automatically switch to home backup power and continue accessing a reliable power source even if the grid is down. 

Grid charging

One of the greatest benefits of using AC-coupled storage is that the batteries can be charged by both solar panels and by the grid. This means that if your solar panels aren’t generating enough electricity to power your home and fill your battery, you can still refill your AC-coupled battery directly from grid power. 

Flexibility

When it comes to installation location, AC-coupled batteries are more flexible than DC-coupled solar batteries, able to be installed farther away from the solar panels if necessary. They’re also relatively easy to upgrade or expand without needing to overhaul the entire system. 

Cons of AC-coupled batteries

There are also some drawbacks of using AC-coupled batteries, which we’ll outline below.

Lower efficiency

AC coupling involves multiple conversions between AC and DC power and a small amount of electricity is lost at each conversion. This makes AC-coupled systems slightly less efficient compared to DC-coupled systems. Generally, the maximum round-trip efficiency of AC systems is about 90%. This means that, for every 10 kWh of solar energy sent to the battery, you'll only get 9 kWh of usable energy at night​. 

See More: 9 Ways to Improve Solar Panel Efficiency and Output

Additional inverter 

While AC-coupled systems are relatively simple to install, especially for retrofits, the overall system can be more complex as both a solar inverter and battery inverter are needed. This can increase material costs a bit, but this is typically offset by the labor savings associated with AC-coupled options. 

Less ideal for new builds

If you’re building a new home and plan on installing solar battery storage at the same time, DC coupled systems become easier to install and you avoid the minor efficiency losses associated with AC coupling. 

What is DC coupling?

DC coupling involves storing electricity generated by solar panels directly into a battery without any conversions. As we mentioned earlier, solar panels generate electricity in DC form. With a DC-coupled system, the power from solar panels is fed straight to the solar battery without any AC/DC conversion. When the stored energy is needed, an inverter transforms the DC power to AC that can be used to power household appliances. 

Unlike AC coupling, DC coupling only converts power once, resulting in less energy loss and more efficiency. This conversion is done using a hybrid inverter, which is basically both a battery charger and a solar inverter in one box. 

Continue Reading: Everything You Need To Know About Solar Inverters

Pros of DC-coupled batteries

Let’s look at the advantages of using DC-coupled batteries.

Greater efficiency

One of the main advantages of DC coupling is its increased efficiency. Since power generated by solar panels is immediately stored in the battery as DC power and only converted to AC when needed, there are fewer conversions involved. This reduces energy loss and makes them more efficient. Unlike AC-coupled solar batteries, which have a round-trip efficiency of around 90%, DC-coupled batteries have an efficiency of up to 97.5%. 

Backup power during an outage

Like AC-coupled solar batteries, DC-coupled systems provide backup power during grid outages, allowing you to continue accessing electricity even if the grid is down. Their greater efficiency, however, means you can maximize energy storage and better use the power generated by your solar panels.

Less components

DC-coupled systems require less components than their AC counterparts, making them more simple to install and configure so long as the installation is happening in conjunction with a solar panel installation. Because they only need one inverter, DC-coupled batteries also usually involve lower initial setup costs. 

More ideal for new builds

If you’re building a new home from the ground up, or installing a new solar system, DC-coupled systems may be more ideal for their greater efficiency and lower upfront costs. 

Cons of DC-coupled batteries

There are also a few drawbacks of DC-coupled systems to be aware of.

Less flexible 

When it comes to installation, DC systems have less flexibility compared to AC systems. The inverter needs to be located close to the battery, giving you fewer options for where to install the system. It can also be more difficult to expand or upgrade a DC-coupled system as they require specific components that might not always be compatible with existing infrastructure. 

Less resiliency

DC-coupled systems only use a single inverter. If that inverter fails, you’ll lose both battery capacity as well as the energy generated by solar power. This makes them less resilient than AC systems.

Challenging to retrofit

One of the main drawbacks of using DC-coupled solar batteries is that they’re more challenging to integrate with existing solar installations. Retrofitting an existing AC-coupled system to accommodate DC coupling is likely to be expensive and complicated, and you’ll need to replace your current solar inverter with a hybrid inverter. This is why DC coupling is more ideal for new builds. 

AC vs DC-coupled battery storage: Which is for me?

Let’s get to the question that’s on most people’s minds: should you choose AC or DC coupling for your solar battery? The exact answer depends on various factors, including your existing infrastructure, efficiency goals, budget, and energy needs. 

To help you make your decision, we summarize the difference between these two storage systems so you can gauge which is more suitable for your needs.

Summary: AC vs DC-coupled battery storage

Both AC and DC-coupled battery systems offer unique advantages and come with their own set of drawbacks. AC-coupled batteries are ideal for retrofitting an existing solar panel system and better suited for those who plan to expand or upgrade their solar battery system in the future. DC-coupled batteries are more suitable for new installations and for those who prioritize maximum efficiency. 

Ultimately, the right option for you depends on your specific circumstances and energy needs. If you’d like some expert guidance in choosing the best energy storage system for you, don’t hesitate to contact the team at Sun Valley Solar Solutions. We can guide you towards the right decision and provide a customized battery storage solution for your needs.