AC Coupling: Adding Batteries to a Grid Tie Solar System
The battery-based inverter is connected to an electrical sub-panel that contains circuits for all the essential loads you want to power during a utility outage.
The unfortunate truth is that the grid is reliable until it isn’t, and being without electricity is a significant modern-day inconvenience. You already have solar power but want to ensure you have power during an outage. Now what?
What if we told you that you can add emergency battery backup without discarding your current system? This process is known as AC coupling.
What is AC Coupling?
AC coupling is a method of adding battery backup to an existing grid-tied solar power system. Your existing system remains unchanged, but when the utility grid goes down, your grid-tied inverter routes power through an additional battery-based inverter connected to energy storage (batteries). This new inverter uses the power stored in the battery bank to supply electricity to your home during a utility outage.
How does AC Coupling work?
The battery-based inverter is connected to an electrical sub-panel that contains circuits for all the essential loads you want to power during a utility outage. When the battery-based inverter detects a grid outage, it automatically disconnects from the grid and begins powering your essential loads using the stored energy from the batteries.
What Happens When the Grid Goes Down During the Day?
When the grid goes down, a transfer switch automatically disconnects your system from the grid and connects it to your essential load subpanel. Your battery-based inverter then begins supplying power from your batteries. The grid-tied inverter perceives this power as "utility" power and continues to operate. During the day, your solar panels keep charging the batteries, ensuring your essential loads remain powered. Once grid power is restored, the battery-based inverter shuts down, and the grid-tied inverter reconnects to the grid, allowing your entire home to be powered as usual.
What Happens When the Power Goes Out at Night?
When the power goes out at night, the process is similar to a daytime outage, but your solar panels aren't available to charge the batteries. Your essential loads will rely on the stored energy in your batteries until the panels can generate power again.
How Big Does My Battery Bank Need to Be?
The size of your battery bank depends on which essential loads you want to keep running during an outage. Essential loads might include the fridge/freezer, well pump, phone charger, CPAP machine, septic pump, and a few lights. It's usually not cost-effective to back up all the loads in a home.
AC Coupling – Is it Really That Simple?
Integrating batteries into a grid-tied system later on is certainly possible, but there are some drawbacks to AC coupling that need to be considered. Sizing a hybrid system to integrate with an existing array comes with design constraints, and sometimes it's not feasible to use your entire solar array for battery charging without significant and often prohibitive expenses.
Conclusion
In summary, it's not ideal to design your solar system this way "out of the gate" because adding a separate inverter and battery bank later means you may only be able to integrate part of the solar array. If you anticipate needing battery backup with your solar system, it's simplest and most cost-effective to install both at the same time. However, if you already have a solar array, AC coupling can still provide the comfort and convenience you need when your grid power fails.
