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AC vs DC-coupled solar battery systems: Pros and cons

If you want to have protection against power outages or plan to live off-grid, you’ll need to add batteries to your solar system. Here the challenge arises: there are DC-coupled battery systems and there are AC-coupled ones. What are those and how to choose between them? In this article, we’ll explore the differences between AC and DC-coupled battery systems and talk about which one is right for you.

Solar battery saves energy for a rainy day

Solar batteries save extra energy from solar panels for use when it's dark, cloudy, during power outages, or when electricity costs are highest. Deciding to add them is a big choice: a battery can increase the cost of your PV installation by up 50%. Sometimes, just connecting your solar panels to the grid without batteries is a better way to save money. 

But if you live in a place with a lot of power outages, getting solar batteries is a smart move, especially if you want to be completely energy-independent. The number of batteries you need depends on how much energy you use and why you want them. The right size for your battery setup is different if you're off the grid or using a mix of solar and regular power.

AC and DC-coupling are two ways to add a solar battery

AC or DC-coupling refers to how solar panels are coupled or linked to a BESS. The type of electrical connection between a solar array and a battery can be either Alternating Current (AC) or Direct Current (DC).

BESS — Battery Energy Storage Systems

In a DC-coupled system, the battery is directly connected to the direct current (DC) side of the power system — the energy from panels goes directly into energy storage. In an AC-coupled system, the energy storage system is connected to the alternating current (AC) side of the power system.

In both configurations, an inverter converts DC output from the batteries into AC before injecting it into the electrical grid or the building's AC distribution system. In an AC-coupled system, an inverter also has to convert AC from a house electric system into DC for a battery. If it feels confusing, don’t worry yet — let’s take a closer look at each type.

DC-coupled battery system in detail

PV panels → controller → battery → inverter → house and grid

A DC-coupled system looks like the most simple configuration. Solar panels produce direct current under the sun. This energy goes into the charge controller that lowers the voltage down to a comfortable level for the battery to charge. This AC either flows into the grid or to your appliances. The charge controller may disconnect the battery if it is full and then energy from panels goes directly to an inverter. 

You can’t pair a standard grid-tie inverter with a DC battery. For this, a hybrid or an off-grid battery inverter is required.

A DC-coupled system is a good choice when you design a solar system with battery storage from scratch. Let’s take a look at the pros and cons of a DC-coupled system.

ProsCons
• Lower cost. DC-coupled systems are cheaper than AC-coupled systems on average. A system requires only one inverter.
• High efficiency. The current in a DC-coupled solar system gets converted only once. This reduces the losses across your installation.
• Simplified Design. DC-coupled systems have fewer components compared to AC-coupled systems, and they are easier to install and maintain. This makes installation and maintenance easier.
• Limited flexibility. Installers have less flexibility than with an AC system, as the inverter needs to be located close to the battery. It can also be hard to add a DC battery to an already existing solar system.
• Backup nuances. In AC-coupled systems, the inverter can often provide backup power during grid outages. In DC-coupled systems, this may require additional components for seamless backup power.

DC-coupled BESS models: LG Chem RESU, PWRCell and more

What are the examples of good DC-coupled BESS on the market? The most popular models include LG Chem RESU, Generac PWRcell and Panasonic EverVolt. Usually, one complete BESS features a capacity of about 8 to 16 kilowatt-hours. Some models, such as Generac PWRcell, employ modular design which means that you can add battery modules to a unit to increase capacity gradually. Note that not every brand allows crossbranding which means using their products together with equipment from other brands.

AC-coupled battery system in detail

PV panels → inverter → house and grid ⟷ inverter ⟷ battery

An AC-coupled battery system is more complicated. DC from solar panels goes to an inverter, turns into AC and flows into the grid or to your appliances. There is also a second inverter in the system. It converts AC from the grid and the first inverter to DC and sends it into the battery.

Although we often use the term “AC battery”, batteries don’t store alternating current. An AC battery has an inverter that converts its DC to AC.

Why so complicated? An AC-coupled battery system is easier to add to an existing solar installation that was not initially designed for energy storage. Standard grid-tie inverters don’t support batteries but with AC-coupled BESS, you wouldn’t have to replace your inverter to get an energy backup.

ProsCons
• Retrofitting. AC-coupled BESS is easy to add to an existing solar panel system. The AC-coupled system works with any type of inverter connected to your solar panels because the battery has its own inverter.
• Power outage protection. Most AC-coupled systems are designed to provide backup power during grid outages. The battery inverter can operate independently of the grid, supplying power to critical loads within the building.
• Lower efficiency. The energy in the AC-couple system gets converted three times: 1) from DC to AC when solar panels produce energy; 2) from AC to DC battery inverter to charge the battery; 3) from DC to AC when you draw energy from battery. Each conversion leads to energy losses.
• Cost. AC-coupled systems cost more than DC-coupled systems as they use multiple inverters.


90-99%

 conversion efficiency of most solar inverters

Tesla Powerwall 2 is the most popular AC-coupled BES

A lithium-ion AC battery by Tesla Powerwall 2 is what you most often encounter as an AC-coupled home energy backup system. Powerwall protects your house from power outages and saves you money during hours of peak utility rates. One PowerWall has a 14kWh capacity. Often homeowners purchase two or three Powerwalls to ensure backup for several days. The battery has 90% efficiency and comes with a 10-year warranty.

We’ve made a video on the question of whether or not getting Powerwall is profitable for you. Turns out, it’s hard to make money off having a home battery. However, customers claim that the battery is high-quality and helps a lot during power outages.

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Andrey Gorichenski
Senior Editor

Andrey had been a news editor and freelance writer for a number of medias before joining A1SolarStore team. Climate change and its impact on people's lives has always been among his interests and it partially explains his degree in Philosophy and Ethics.

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