grid tie

How Do I Integrate a Battery Backup with a Grid-Tie Solar Power System?

One of the most common questions asked by customers is how to integrate a battery backup solution with an existing grid-tie system. As designed and required by law, grid-tie systems shutdown during a grid power outage. The main reason is to make sure solar panels are not back-feeding power into the grid while line workers are attempting to make repairs. With those details being known, customers want to maintain some level of power during a grid-outage for powering essential appliances or critical loads. Resolving that issue requires integrating a battery backup alongside your grid-tie system that does not feed power back into the grid.

There are a few different ways to achieve it. One of the more common methods is called AC Coupling. This is a system configuration that involves adding a battery-based inverter and a battery bank into an existing grid-tie system as well as a critical loads panel. A critical loads panel is needed to power all the devices and appliances needed to remain powered during a grid outage. The battery-based inverter and the critical loads are connected to the critical loads panel. AC Coupling requires that the output of the grid-tie inverter also be connected to the same critical loads panel. This design places the battery-based inverter output and the grid-tie inverter output on a common bus or loads panel resulting in the two being coupled together hence the phrase “AC Coupling”.

In this configuration, when grid power is present the solar panels are feeding power to the grid as normal which covers the loads on the critical loads panel. Any excess production of power will follow a sequence of events to make sure all loads are satisfied before feeding back to the grid. The power from the solar panels will first power any critical loads you have established on the critical loads panel. Once those loads are satisfied, excess power coming from the solar array will then charge your battery bank. Once the critical loads and the battery are satisfied and if the solar panels are still producing an excess of power, that power will be sent back to the mains load panel and would offset any loads present. Beyond that, if the solar panels are still producing an excess of power, the system will feed power back to the grid as designed.

On the contrary, when there is a grid outage and grid power is lost the loads on the mains power panel are no longer being fed. Excess power is no longer being sold back to the grid. The system at that point is being contained to the loads on your critical loads panel. During the grid outage, the battery-based inverter is still producing power and sending power to your critical loads panel. The grid-tie inverter sees the voltage and frequency from the battery-based inverter and is somewhat “tricked” into thinking that the grid is still active which results in the solar array being able to produce power and cover the critical loads and charge the batteries. The batteries will then be able to power the critical loads at night given the grid is still offline due to the power that was stored from the solar panels during the day.

In closing, AC Coupling offers a fail-safe option to power critical appliances like freezers and water pumps if grid power goes down and allows the system to back-feed the grid when grid power is active while ensuring your battery bank is fully charged.

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