AC or DC coupling refers to the way in which solar panels are linked to the BESS (battery energy storage systems). Here we compare the pros and cons of each.
Solar panels can be coupled or linked to a battery either through alternating current (AC) coupling or direct current (DC) coupling. AC current flows rapidly on electricity grids both forward and backward. DC current on the other hand, flows only in one direction.
DC current is what is generated by solar panels and stored by batteries, but because appliances use AC current (just look at the AC circuit board you have in your home), the current must be converted via inverters.
In the past, AC-coupled BESSs were most often used with residential and commercial solar installations, and DC-coupled systems were used for remote and off-grid installations, but more options for DC-coupled systems have become available. Equipment manufacturers are developing streamlined and standardized power electronics equipment for DC-coupled BESSs.
Over the past decade, inverter technology has advanced and resulted in the development of new AC-coupled and DC-coupled systems. Let’s find out more.
- What Are AC-Coupled Systems?
- What Are DC-Coupled Systems?
- What Are the Advantages of AC-Coupled Battery Systems?
- What Are the Disadvantages of AC-Coupled Battery Systems?
- What Are the Advantages of DC-Coupled System?
- What Are the Disadvantages of DC-Coupled System?
- Should I Use an AC- or DC-Coupled System for My Solar Plant?
- RatedPower’s Platform Ensures the Efficiency of Your Plant
What Are AC-Coupled Systems
In AC-coupled systems, there are separate inverters for the solar panels and the battery. Both the solar panels and the battery module can be discharged at full power and they can either be dispatched together or independently, creating flexibility in how the system operates. The solar panels and battery can either share an interconnection to the grid or run on separate interconnections.
AC-coupled battery energy storage system diagram. Source: RatedPower
AC BESSs comprise of a lithium-ion battery module, inverters/chargers, and a battery management system (BMS). These compact units are easy to install and are a popular choice for upgrading energy systems, and the systems are used for grid-connected sites as the inverters tend not to be powerful enough to run off-grid.
It’s worth noting that because both the solar panel and the battery are DC-current compatible, the current will need to be converted three times in an AC-coupled system.
What Are DC-Coupled Systems?
DC-coupled systems typically use solar-charge controllers, or regulators to charge the battery from the solar panels, along with a battery inverter to convert the electricity flow to AC.
DC-coupled battery energy storage system. Source: RatedPower
The solar panels and battery module use the same inverter and share the grid interconnection, reducing the cost of equipment. This also reduces power losses from inverting the current and running separate interconnection lines to the grid, as the solar array and battery are dispatched as a single facility. But this offers less flexibility than an AC system.
What Are the Advantages of AC-Coupled Battery Systems?
There are several benefits to using an AC-coupled BESS for your solar plant, including:
- Retrofitting: AC-coupled batteries are easy to install on an existing solar panel system and more can be added to expand capacity.
- Flexibility: Installers are not restricted in where the
inverters and batteries can be located. AC coupling works with any type of inverter. - Resiliency: The flexibility to install multiple inverters and batteries in different locations helps minimize the risk of an outage if an inverter fails. Having multiple inverters provides more combined power and battery faults do not have an impact on power generation.
- Versatility: AC-coupled systems enable batteries to charge from the grid as well as the solar panels, so if the solar panels are not generating enough electricity, the battery can still charge from the grid.
What Are the Disadvantages of AC-Coupled Battery Systems?
- Cost: AC-coupled systems cost more than DC-coupled systems as they use multiple inverters.
- Lower efficiency: The stored energy is converted three times; from the DC current to AC current to supply the building, and then back to DC current to the battery, and again back into AC. Each conversion results in a small amount of energy loss.
- Supply limitations: AC BESSs are not designed to be used off-grid and as they are transformerless, they cannot manage the surge loads from multiple appliances.
What Are the Advantages of a DC-Coupled System?
Where AC-coupled systems suffer in terms of efficiency and cost, DC-coupled systems have the advantage:
- Affordability: DC-coupled systems tend to be cheaper than AC-coupled systems, as the solar panels and battery use a single inverter and less extra equipment such as voltage transformers and switchgear.
- Higher efficiency: Unlike AC systems which convert the current multiple times, DC BESSs only convert the current once, reducing energy losses and making them more efficient.
- Oversizing: DC-coupled systems allow solar panels to generate more electricity than the inverter rating. The excess energy can be used to charge the battery, an EV charger or a water heating system, whereas in an AC-coupled system, the energy is lost.
What Are the Disadvantages of a DC-Coupled System?
- Limited flexibility: Installers have less flexibility than with an AC system, as the inverter needs to be located close to the battery.
- Less resiliency: With a single inverter in a DC-coupled system, if the inverter fails, the solar power as well as the battery capacity is lost.
Should I use an AC- or DC-Coupled System for My Solar Plant?
AC-coupled systems are the preferred option for larger and utility-scale plants. That’s because while AC-coupled systems are slightly less efficient at charging batteries (90-94% vs. 98% achieved by DC-coupled), they are far easier to install, especially into an existing system.
That said, whether AC-coupled or DC-coupled is the best solution for your PV plant design will be project specific. You can use a PV plant software solution to run several simulations and identify the best option.
Simplify Battery Energy Storage System Designs
With RatedPower’s BESS design tool, you can seamlessly integrate battery storage with PV plants, supporting both AC- and DC-coupled configurations. Quickly optimize storage capacity, generate full engineering documents, and compare AC vs. DC hybrid PV systems for cost and production efficiency. Automate your BESS layout generation, container placement, and power factor adjustments while leveraging a comprehensive equipment library. You can also design standalone BESS projects with refined calculations for topography, earthworks, and cable sizing.
Take a product tour today and learn how Enverus RatedPower can revolutionize your BESS design.
