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AC Coupling Explained

AC Coupling Explained

AC Coupling is really just adding BATTERY BACKUP to an EXISTING GRID-TIE PV SYSTEM.

Most grid connected solar systems do not include a method for storing the power that the system is producing and will shut down during a utility power outage. When the grid goes out, so does your solar system. By adding batteries, you can store the energy that the solar array is producing and use it during an electrical outage. What makes Backwoods Solar and other off-grid focused companies uniquely qualified to design and sell AC Coupled systems is that we truly understand battery storage systems. Many grid-tie solar installers haven’t worked with batteries before, and many choose to continue to not work with batteries. That doesn’t mean that it has to be daunting for the homeowner.

Backwoods has made this easy by designing several kits with Magnum PAE inverters and maintenance-free sealed AGM batteries. Each system is based on the length of time and size of the loads that you want to back-up during an outage. Most of the components come prewired, so with the addition of a critical loads sub-panel, you can start storing energy for use when you REALLY need it, when the lights go out everywhere else.

The systems continue to work during daylight hours, charging the batteries and providing power to the back-up loads throughout the black-out period. So day or night, you will have power for use in your home.

How AC Coupling Works

AC Coupling is the process of tying in an additional battery based, off-grid inverter/charger. The inverter/charger works alongside your existing grid-tie string or micro-inverters, keeping the system running during a power outage.

A critical loads sub panel is necessary for this application. The "critical loads" sub panel is an additional breaker panel that is separate from a building's main breaker panel, designated only for appliances backed up during power outages. When we design and size the back-up portion of an AC Coupled system, we look at it similarly to a stand-alone off-grid system or micro-grid system. The system becomes “islanded” during an outage, functioning very similarly to a traditional off-grid system without grid input. There are a couple of unique differences with an AC Coupled system in terms of sizing to match the existing grid-tie array and the lack of charge controllers. We go into more detail about this further along in the article.

When the grid is active, power is flowing through the Magnum inverter/charger to the batteries to maintain their charge. Your grid connected inverter will function normally, selling power back to the grid and off-setting the loads in both the main panel and sub panel.

When a black-out occurs, the Magnum inverter/charger seamlessly begins to invert power from the battery bank to power only the loads in the critical loads sub panel. The grid connected inverter cannot sense the disconnect of the main power supply due to the Magnum being online and continues to feed power to the system.

Adding a Grid-Tie Retrofit Kit to your existing grid-tie system doesn’t require any change to the existing solar wiring. The most complicated wiring will be the moving of the loads you want to power during a utility outage to breakers to the new back-up “critical loads” sub panel. The AC output of the existing grid-tie system will also be moved to critical loads sub-panel.

Backup batteries must be protected from overcharging during grid outage periods. Magnum Energy PAE inverters can be programmed to change their output frequency slightly by setting the battery type for “Custom Settings”. When in this mode, the Magnum inverter will shift its frequency to 60.6 Hz when the battery voltage gets to 4 volts higher than the absorb-voltage in a 48 volt system. This will cause the grid-tie inverter to turn off for 5 minutes, stopping the battery from charging and allowing the loads to be powered from the batteries. After 5 minutes the grid-tie inverter turns on again and begins charging the batteries. This process repeats as long as the grid-tie system is making more power than the house loads are using.

As a secondary safety mechanism, Backwoods Solar provides a relay that communicates with the Magnum router to disconnect the grid-tied inverter and the solar array when the grid is inactive, the sun is shining and more power is being produced than consumed through the critical loads sub panel. As in the method above, when the battery voltage climbs to a pre-set voltage the relay will trip, disconnecting the grid-tie inverter. When the loads pull the voltage of the batteries down again to a lower pre-set voltage the grid-tied inverter and solar array will come back online (there is a 5-minute timeout that is preset in all grid-connected inverters that will need to progress before coming back online).

How Do I Set Up a Critical Loads Sub Panel?

First, determine what things that you can’t live without during a power outage. Most of the time it would be a refrigerator, some light and power circuits, and perhaps a well-pump, etc. The minimum amount of loads that you transfer results in a reduction in the amount of batteries that are required to power the loads.

See our load evaluation worksheet, which can be used to determine the size of the loads that you want to back up.

Once you have determined the loads that you want to back-up and the amount of time you want to be prepared to back them up, we can help you determine the size of the battery required. This type of work should be done by a qualified electrician or electrical contractor and permitting may be required in some jurisdictions.

Sizing the Battery Bank

First think about the length of time that your power is typically out, a couple hours to multiple days. This is going to determine; along with the critical loads size, the total capacity needed within the battery bank. In an effort to simplify the process, all of our systems battery banks are shown in watt-hours (Wh).

The total amount of watts your critical loads consume times the length of time that you want to back-up those loads (duration of outage) will determine the size of the battery bank for your system. Plan for the worst case scenario in terms of the weather. Although the sun may be shining during the daylight hours of an outage, most of the time we experience outages during severe weather storms that have low-light conditions. This means that you may not experience the full output of power from your solar array.

Sizing the AC Coupled, Retro-Fit Overall System Size

Which AC Coupled system is for you? The Magnum inverter needs to have a 10% higher wattage than the production wattage of your solar array after derating for system inefficiencies. If you have multiple grid-tied string inverters wired in parallel within your system, it is possible to only utilize one of the inverters and a portion of the array for back-up purposes. For systems with micro-inverters, if you have multiple strings of micro-inverters wired into your main panel, it is possible to only utilize the same methodology and only use a portion of the array and inverters for back-up purposes.

Still Need Help?

As with all of the systems that we design and sell, you can call us any time to review the details and receive a FREE custom system design for your application. We will provide support before, during and for the life of your AC Coupled system. Just call or email us!