AC-Coupled System
An AC-coupled system is a solar-plus-storage configuration where the solar inverter and the battery inverter operate independently but connect on the AC side of the electrical system. Instead of the battery being charged directly from the solar array’s DC output, an AC-coupled design routes solar energy through the inverter first, converting it into AC before the battery inverter converts it back to DC for storage.
This architecture is widely used in residential, commercial, and microgrid applications because it is flexible, retrofit-friendly, and compatible with most existing grid-tied solar systems. Installers often choose AC coupling during upgrades or battery add-ons because it does not require replacing the existing PV inverter.
AC-coupled systems play a key role in workflows built using tools like Solar Designing, helping designers model production, load shifting, and backup behavior with accuracy.
Key Takeaways
- An AC-coupled system connects solar and battery inverters on the AC side.
- Ideal for retrofits, backup systems, and flexible hybrid configurations.
- Slightly lower round-trip efficiency than DC-coupled systems.
- Allows solar and battery inverters to operate independently.
- Supports residential, commercial, and microgrid applications.
- Easily modeled and optimized using SurgePV design tools.
What Is an AC-Coupled System?
An AC-coupled system is a hybrid solar system where both the solar system and the battery system convert energy through separate inverters and communicate on the AC bus. This differs from DC-coupling, where solar PV charges the battery directly through a shared DC bus.
In AC coupling:
- The solar inverter converts DC → AC
- The battery inverter converts AC → DC (to charge the battery)
- Both inverters operate independently
- They share power on the main AC distribution panel
This architecture is highly versatile, making it ideal for:
- Retrofit battery installations
- Systems requiring backup power
- Homes or businesses with existing grid-tied PV
- Complex loads and microgrids
- Projects requiring flexible system expansion
Related engineering concepts include Solar Inverter, Stringing & Electrical Design, and Load Analysis.
How an AC-Coupled System Works
1. Solar panels generate DC power
Panels send DC electricity to a standard grid-tied solar inverter.
2. The solar inverter converts DC → AC
Converted AC power is fed into the building’s main service panel.
3. The battery inverter handles charging and discharging
The battery inverter (also called a hybrid or backup inverter):
- Converts AC → DC to charge the battery
- Converts DC → AC to supply backup loads
4. AC power flows through the main panel
Both systems “meet” at the AC bus, enabling coordinated operation.
5. Backup mode activates during outages
The battery inverter isolates (“islands”) the home or building and provides AC power from stored energy.
For voltage configuration considerations, see Voltage.
Types / Variants of AC-Coupled Systems
1. Grid-Tied AC-Coupled System
Most common design. Solar and battery systems share the main AC panel while remaining grid-connected.
2. Backup-Ready AC-Coupled System
Includes a critical load panel and automatic transfer switch (ATS) for backup during outages.
3. AC-Coupled Microgrid
Used for remote sites, farms, commercial buildings, or community energy systems where solar, batteries, and generators interact through AC coupling.
4. AC-Coupled Retrofit System
Allows batteries to be added to an existing grid-tied PV system without replacing solar inverters.
How an AC-Coupled System Is Measured
Solar designers typically measure:
1. AC System Power (kW)
The rated AC power output of solar and battery inverters.
2. Battery Charge/Discharge Power (kW)
Determined by the battery inverter’s AC rating.
3. Storage Capacity (kWh)
Defines how long loads can be supported.
4. Round-Trip Efficiency (%)
AC-coupled systems usually have slightly lower efficiency due to double conversion:
DC → AC → DC → AC
5. Load Coverage (%)
Represents how much of the electrical load the system can support.
To analyze load behavior, see Load Analysis.
Typical Values / Ranges
Efficiency and system design vary by region, AHJ requirements, and local grid standards.
Practical Guidance for Solar Designers & Installers
1. Use AC coupling for retrofit battery installations
No need to replace the customer’s existing solar inverter.
2. Size the battery inverter for peak backup loads
Large HVAC or motor loads may require higher surge capacity.
3. Account for round-trip efficiency losses
AC coupling adds additional conversions, impacting net storage yield.
4. Verify interconnection with AHJ and utility requirements
Grid-tied systems must adhere to NEC, IEEE 1547, and local utility guidelines.
See AHJ Compliance.
5. Use professional software for layout and modeling
SurgePV tools streamline hybrid system layout, cable routing, and performance modeling:
6. Incorporate shading studies for accurate performance
Use Shadow Analysis to understand battery-charging opportunities.
7. Educate customers on backup capabilities
Not all loads can be backed up unless the system is sized properly.
Real-World Examples
1. Residential Retrofit System
A homeowner adds a battery to a 7 kW existing solar system.
An AC-coupled battery inverter is installed without modifying the PV array or inverter.
2. Commercial Backup-Ready System
A retail store installs a 50 kW solar system and 30 kW AC-coupled battery inverter to support refrigeration and emergency circuits during outages.
3. Remote Microgrid
A farm uses two AC-coupled inverters—one for a 20 kW solar array and one for a battery bank—to run irrigation pumps even during grid instability.
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