AC Coupling – Adding A Battery Back-up Package To Your Existing Grid-tie System

Buyer’s Remorse.  …8220;Why, oh, why didn’t I buy a battery backup PV system?…8221; Or, …8220;I hate looking at my new PV system shut off when the grid’s down.…8221;

You may already have a fabulous grid-tied solar system installed on your house, making power when the sun is shining, and turning back your meter. That is absolutely fabulous! Grid-tied systems are the most cost effective and easiest systems to install. Until the day the grid goes down, and your house is without power while your neighbor with the loud, smelly generator is sitting in their house drinking frozen margaritas and watching TV. What to do now? No, you can’t run an extension cord to your neighbor’s house and hope they don’t notice. Electrical code requires that our grid-tied system shut down if the grid goes out to prevent accidentally electrocuting the linemen while they are working on a power lines to restore power.

You may think you are locked into the option you bought, or have to get rid of a lot of your equipment and start over.  Not so. There is an increasingly popular configuration called AC Coupling.  AC Coupling uses your existing system to feed into a grid-tied, battery backup (GTBB) inverter/charger to charge the battery bank. Here’s how it works.  You keep your existing system exactly as is, except you add a GTBB backup inverter/charger and battery bank.  The battery bank doesn’t need to be huge, just large enough to run your critical loads needed during a power outage; the fridge, well pump, furnace or boiler system’s electrical components, a few lights, and maybe recharge your cell phone or iPad to let friends and family know you are OK.

A GTBB inverter/charger connects to a new subpanel called the “critical loads panel”.  It’s the breaker box that the most important circuits are connected to. The GTBB inverter is also connected to the grid. When inverter detects that the grid is out, it turns off any connection to the grid, and only powers the items wired into the critical loads panel.  Therefore it complies with the requirement to not send power down the lines, but allows you to have some power in your house. In an AC Coupled system, your existing grid-tied inverter is also connected to the critical loads panel. When the grid is up, it sends power through the GTBB inverter, out to the rest of the house, and any excess power gets sent to the grid to spin the meter backwards. But when the grid goes out, the GTBB transfer switch flips, and it turns off any connection to the grid, and only to the critical loads panel.  Since the grid-tied inverter is also connected there, it instantly sees the output from the GTBB inverter, and thinks it’s the grid. The inverter doesn’t turn off.  It keeps sending solar power to the GTBB AC charger component, to charge the battery bank that the GTBB inverter is getting its power from. So you are powering your critical loads off the battery bank, while the solar panels are recharging the batteries during the day.  Once the grid comes back up, the transfer switch flips back, and the grid-tied inverter goes back to powering the whole house and spinning your meter backwards.

Certain restrictions apply depending on which inverters you have, give us a call to see if AC Coupling is a good option for you.