Many homeowners want security and protection in case the grid goes down, and battery backup systems are a great solution to bring this peace of mind.

Battery Backup: Power When it's Needed Most

Greg Smith | SMA

Superstorm Sandy devastated the east coast, the Super Bowl was dramatically interrupted, and most recently Nemo left nearly 700,000 people without power in the dead of winter. These events highlight the importance of battery backup when electricity is needed most, and present a significant opportunity for growth in the solar industry.

When the grid goes down and the power goes out, many people think that PV systems should kick into gear and act as a source of emergency power. However, due to safety regulation requirements, grid-tied PV systems also stop working in the event of a grid outage.

But there is an option that will supply power in an emergency situation: Battery backup power.

A Simplified Solution: AC Coupling

AC-coupled battery backup systems are a cost-effective, accessible way for people to equip their homes or businesses with a backup power source in the event of a grid outage. AC-coupled systems provide flexibility so that even years after the initial installation, if needed, a larger battery bank can be installed and a generator or more PV can be added without additional training or components.

Battery Backup Hybrid System without PV Battery Backup System without PV

Designing an AC-coupled system does not require specialized knowledge of electronics, and commercially available components can be used since they are readily available.

System Sizing

Instead of backing up an entire house, it’s usually most effective to protect certain electrical functions. Critical load breakers are migrated from the main service entrance to a special subpanel. The solar inverter breaker and the load breakers are placed into this subpanel, which is called a critical loads panel. The battery inverter also feeds this critical loads panel when the grid is lost. AC-coupled battery backup systems use energy from inverters to directly power essential loads when the grid goes down. Any excess power is sent to the battery inverter to charge the battery bank.

Battery Bank: A load calculation worksheet is essential to begin this process. The worksheet lists all the loads that will be connected to the protected loads panel, which will continue to operate when the grid goes down. Every load must be logged, along with how much power it uses and how long it will run per day. The next step in the sizing process can take place once the total watt hours have been calculated.

PV and the Inverter: A common rule of thumb with battery inverter sizing is 1 W of inverter per 1 W of backup load. The nominal output rating of the inverter should be used as a general guideline. This is an important step, especially if there is a requirement for sustained power output. If the average power consumption is 8 kW, a 6 kW battery inverter will not be able to keep up and other sources of power will be necessary.

The proper sizing of the inverter is also crucial since it will be the only source of power available to charge the batteries after loads have been supplied, assuming a generator is not used. Since the load profile rarely coincides with the power produced by a PV system during the day, the system should be sized to charge the batteries to an acceptable state-of-charge to account for nighttime needs. A good guideline is to have 1,000 W of PV per 100 Ah of battery capacity.

However, the amount of sunlight available on the worst day of the year must be evaluated for PV sizing since it is more likely that the power would go out at this time. The last item to consider in this application is the array size. The battery bank will need to be charged enough during the day to supply nighttime and early morning loads. 

Generator Sizing (Optional): Not all backup systems will utilize a generator, but it’s a good way to ensure additional security and independence in the event of a grid outage. The generator should be sized to handle the worst case scenario: all loads on, no PV. A practical guideline for generator sizing is 80 to 120 percent of the battery inverter at nominal capacity; however, it is wise to follow the PV rough sizing rule at twice the battery inverter nominal power.

Equipment Selection

Choosing the right equipment is a critically important step in designing a backup system. By their very nature, these systems will be depended upon to perform in harsh environments with little room for error. Be sure to research battery backup inverters and learn as much as possible about the available options to ensure that the right choice is made.

Power Customers Can Count On

Providing backup power to customers makes good business sense. This is an opportunity for solar professionals to assist those in weather-impacted areas. Later on, should they decide to add PV or a generator, they can call on you to add to the system and trust that you will offer the same reliable customer service. It is also a good way to reach out and provide additional security to those already enjoying the financial benefits of grid-tied PV.

Many homeowners want security and protection in case the grid goes down, and battery backup systems are a great solution to bring this peace of mind. With a better understanding of system design, solar professionals can reassure customers that their power will stay on – even when the unexpected happens.

Greg Smith is a technical training specialist at the SMA America Solar Academy. For more information, please contact

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