Adding Battery Backup to Your PV System with AC-Coupling

Intermediate

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AC-Coupled PV equipment
Large, AC-coupled PV system.
This AC-coupled system, installed by Gulf South Solar in Baton Rouge, Louisiana, offers backup power—an important feature to have in a place frequented by tropical storms.
This AC-coupled system uses an SMA America Sunny Boy batteryless inverter (left) paired with and controlled by a Sunny Island inverter-charger (right).
MidNite Solar offers prewired AC-coupling solutions for battery-based inverters.
AC-Coupled PV equipment

Batteryless grid-tied PV (GT PV) systems are affordable, efficient, and simpler than their battery-based counterparts. But when the grid goes down, the system goes offline, leaving the homeowner without electricity. Fortunately, there are solutions for those who don’t want their electricity access disrupted by utility outages.

There are two different approaches to integrating battery backup into an existing GT PV system—DC-coupling or AC-coupling. Both involve adding a battery bank and a battery-based inverter-charger; plus the associated disconnects and overcurrent protection, and a backed-up (or “critical”) loads subpanel for the appliances that need to continue operating during an outage.

The DC Approach

The conventional battery-based PV system is “DC-coupled”—all power generation is on the DC side of the system. All sources operate at the same system input voltage, typically 12 to 48 VDC.

This strategy avoids the compatibility, technical, or warranty issues of an AC-coupled system (see below). Benefits include having the preferred three-stage (tapered off) battery charge control during all conditions and being able to recharge the batteries during a utility outage, even after they have been drained beyond the low-voltage cut-out point.

To minimize wire size, which is more economical for long wire runs, PV arrays can be wired for higher DC voltage (commonly 150 VDC but up to 600 VDC) using a step-down charge controller to convert to battery bank voltage. Energy stored in the battery bank is converted to AC by an inverter–charger, which is usually set up to provide energy first to a critical loads sub-panel, then to the grid through the main distribution panel and utility meter. In the event of a grid failure, the critical loads will continue to be powered by the inverter.

DC-coupling a grid-tied system can take two approaches. Both replace the existing batteryless inverter with a battery-based inverter-charger.

The first approach adds a standard, low-voltage (150 VDC maximum) charge controller. Depending on the number and size of the PV modules, adding or eliminating PV modules may be necessary to meet the input voltage requirements of the charge controller. The PV array’s wire size may also need to be increased (lower voltage means higher amperage, necessitating larger wire to carry the current).

The existing high-voltage (up to 600 VDC) PV array will have to be reconfigured for the charge controller’s DC input voltage range. A combiner box and overcurrent protection (fuses or circuit breakers) may need to be installed if there are three or more paralleled series strings.

The second DC-coupling approach uses a higher-voltage (up to 600 VDC) charge controller. This method eliminates rewiring the array and adding a combiner box.  While you may save labor and equipment costs, the more expensive high-voltage charge controller may negate these savings. Lower-voltage charge controllers can be about $1,000 less than their higher-voltage counterparts.

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Comments (4)

Buckville Energy's picture

Nice rundown on how to do it. Adding battery backup to existing direct grid tie systems has become a huge issue here in CO lately, and AC-coupling issues vary so much by manufacturer as to how they deal with battery charging.

cmeans's picture

This is a great rundown of two common methods for behind-the-meter storage. The third, and likely most compelling option, is to use an islanding inverter that directly integrates with batteries. It would be great to see a follow-up article about next-generation inverters that perform this function, of which there are several noteworthy brands available.

WILSON's picture

thanks for knew generation technology

Diego Matter's picture

Thank you for this very comprehensible article. I`m sure many solar system owners will want to upgrade their systems in the future.

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