A standard grid-tie solar PV system will not power your home during a utility outage, so in areas where blackouts and weather-related outages are common, a battery backup system may add value and convenience.
Your choices include using a grid-tie with battery backup system, or an AC Coupled system. AC coupled systems may be used with an existing grid-tie system with battery backup, or with a system that is completely off-grid.
Above all, it is important to note that typically, these backup systems are not designed to meet the power demands of your entire home, and while they can be modified to meet these demands, the complexity of the job substantially increases the cost.
Grid-Tie with Battery Backup
However, designing and sizing a grid-tie system with a battery backup is much more complex than designing a standard system without energy storage. This is due to the system performing two different functions.
The first is offsetting the power purchased from an electric utility, as standard grid-tie systems do. The second is providing emergency backup power during potential power outages. Each of these purposes have completely separate design considerations and require a different set of calculations, which is why it is best to consult with a professional, especially if you are unfamiliar with the system.
Both parts of the system perform a necessary function. While the grid-tie portion of the system is designed to offset energy consumption during peak sun-hours, which vary depending on where the PV array is located, the battery backup is instead designed to meet the power demands of critical loads that need to operate during a grid outage.
It is important to remember that these types of systems are designed to run only specific circuits located in a separate sub-panel, and as such, are not designed to power the entire home. While this can be done, powering the entire home increases cost and complexity of the system dramatically.
Like a grid-tie with battery backup, an AC-coupled system is form of battery-based system. It can be used in either a grid-tie system with a battery backup application, or in a completely off-grid system.
Rather than using a battery charge controller with the PV array, AC-coupled systems utilize standard grid-tie inverters that produce AC power that can then be “sold” back to the utility grid while the grid is connected. You also have the option for a separate battery-based inverter to use the AC power to charge a battery bank during a grid outage.
In addition to a standard grid-tie inverter, a second bidirectional battery based inverter is used with a battery bank to provide AC power during an outage. During normal operation, when the grid is up, the power from the PV array and grid-tie inverter passes through the sub-panel and battery inverter’s built-in AC transfer switch and then onto the utility main panel.
From the main panel, it is either consumed by house loads connected there or exported to the grid. Should a grid outage occur, the grid-tie inverter will automatically shut off, while the battery-based inverter switches off the grid connection and begins to power the loads in the protected loads panel, using energy drawn from the battery bank.
Because the grid-tie inverter is connected in this sub-panel, it will detect the AC power from the battery inverter and will turn back on after a short 5 minute delay. After this delay, the power output will be used by the protected loads connected to the sub-panel, or they may be used to charge the batteries via the battery-based inverter or charger.