I have two solar-electric systems at my home in Puerto Rico. One system, installed in 2000, has 26 12-volt, 60 W modules in two subarrays connected to two C-60 charge controllers, with 10 T-105 batteries and a 2,500 W inverter. The other system, installed in 2014, is grid-connected, with 12 24-volt, 240 W modules with Enphase 215 microinverters. I’m wondering if there’s a way to use the old system to power the new one when the grid goes down?

Alberto Marty • Lajas, Puerto Rico

The simplest and least expensive way to provide battery backup is an automatic transfer switch between your off-grid inverter and main AC panel. This would power that panel and disconnect the new PV array during a blackout. But if your total loads are close to (or more than) 2,500 watts, you’ll need another option.

To avoid the large-loads problem, consider a backed-up loads subpanel to power only your most important loads—such as refrigeration, lights, etc.—from the battery bank. It might reduce your loads enough that the 2,500 W inverter could cover them.

If the loads exceed inverter capacity, you might be able to use AC-coupling. These systems rely on the capability of “multimode” inverters to supplement your battery-based system with AC from the second, grid-tied system when the grid goes down. Unfortunately, it is unlikely that your older inverter has AC-coupling capability, so you’d need to purchase a modern multimode inverter with split-phase 120/240 VAC output to accommodate the microinverters’ 240 VAC output.

Another issue is controlling battery charging from the AC-coupled PV array. Your existing controllers can manage charging only from your old array—not from your newer microinverter-based system. When the grid is out, multimode inverters can trigger your microinverters to shut down the new array when the battery bank is fully charged.

However, as a safeguard, Enphase requires a “blackout relay,” which completely cuts off the multimode inverter and forces the microinverters to shut down. This results in one-stage charging, with the inverter/charger shutting down for 5 minutes, then restarting for 5 minutes. If grid blackouts are infrequent and short, little or no damage to your batteries is likely to occur under this setup. But best battery life is achieved with three-stage charging. If blackouts are frequent and long-lasting, a standard diversion-load controller that “dumps” extra incoming energy to heating elements when your batteries are full is recommended. However, both the heaters and controllers need to be sized to handle your new array’s entire output.

With the advent of new, high-tech battery types and increasing interest in supplying power during outages, AC-coupling is becoming popular and the technology is rapidly becoming more sophisticated and easier to implement. Best of luck with your project!

Dan Fink • Buckville Energy Consulting

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