Should I add Battery Backup for My Solar Panels?

Types of Battery Backup

Are you considering adding a battery backup option to your solar system? If you want solar installed on your home or business, there are some different battery backup systems to consider based on your specific needs. The expected return on investment and what loads you want to backup both play a critical decision in the type of system you want to install. Here, we’ll discuss the main types of solar battery backup systems and provide a comparison between them.

The majority of today’s solar systems installed are grid-tied. They are meant to offset electrical usage and send excess production back to the utility grid, where your power company will credit the surplus energy. The credits earned can be used at night or during periods of no sun. Throughout the year, this process of sending electricity back to the grid for credits is called “net-metering.”

Sized correctly, a net-metered system should zero out a customer’s utility bill. When temperatures are cooler and HVAC usage is minimal in the spring and fall months, solar systems will overproduce and send much of the energy to the power company for credits. So for these months, the electricity bill is zero. Then during the summer, when HVAC usage soars, the solar system will underproduce…but you will use those credits that have been banked during the cooler months, effectively zeroing out your electric bill in those months too.
(Although, not always read our article on why your solar system might not be net-zeroing out your bill.)

Brands of Grid-Tied Systems

The two brands of systems Sun City installs for grid-tied solar power are Enphase and SolarEdge. Both are great for maximizing solar production and getting the best ROI to pay back system costs in the least amount of time. Sun City systems are sized to deliver enough energy to offset your annual electricity consumption. Equally important, the system can sell back power when needed. In addition, they both have battery options that allow solar power to operate when the grid goes down.

(For more info on how solar interacts with the utility grid, read our article about solar power and batteries.)

Battery Backup Options

The battery backup options with both systems are integrated and sleek. Each system has a proprietary battery; for Enphase, it’s the Encharge battery, and for SolarEdge, it’s the LGChem. These batteries are Lithium-Iron-Phosphate (LiFePO) technology, which is the standard now for solar battery backup due to its battery stability and longevity. Unfortunately, you can’t use a third-party battery with these systems. The systems also include an integrated Auto Transfer Switch (ATS) that automatically isolates (or “island”) the house from the grid when the power goes out so the solar/battery system can power the loads. But this ATS is a separate piece of equipment that you need to purchase and have installed.

Generac Power Cell

Another option for grid-tied solar is the Generac Power Cell. The GPC is a hybrid system…made by Generac for backup power but using an inverter sized for entire home consumption to offset your electricity usage. Here the ATS is integrated with the inverter.

The downside to these types of systems is the battery size. You can’t get much output from a single Encharge or LGChem battery. In addition, you are limited on the number of batteries you can combine, limiting bank capacity and throughput of energy. These systems are good at providing backup for a smaller critical load panel, but not necessarily an entire house. The primary goal of grid-tied solar is to sell back energy. Backup is secondary. If you are planning to go off-grid, these are not the systems we recommend.

True Battery Backup Based Solar System

The second type of backup system is a “true” battery-based solar system. These systems use “Hybrid” inverters because they are both inverters AND chargers: they invert the DC voltage to AC and power your loads and can also take AC voltage from the grid and invert to DC voltage to charge the batteries. The Hybrid systems can also isolate the backed-up loads from the grid to create the necessary “island” to enable the solar to power the loads while the grid is down.

The brands of Hybrid systems that we’ve installed include Schneider Electric, Outback, and Sol-Ark. We’ve also worked on many others since we are the off-grid battery experts in our area. These systems are also grid-tied when utility power is available. They can use grid power to keep the batteries charged. Because they are connected to the utility grid, they can also sell excess production to the power company.

Purpose of Solar Production

Solar production is first meant to keep the batteries charged (you want to have a full power bank when the grid goes down!). The only secondary will the system sell back, and then there is usually a limit to how much can be sold. The Hybrids are sized for the backup load, not necessarily for your annual electricity consumption, meaning the amount of power sent back to the POCO is smaller than the grid-tied system options described earlier.

The battery options for these systems are countless, however. We’ve installed standard lead-acid, absorbed glass mat (AGM), and LiFePO. We’ve also installed Iron-Nickel technology and even put 8 LiFePO batteries in parallel in the same bank. You can make the battery bank as large as the Hybrid inverters can handle. This option is not something you can do with the grid-tied systems.
With a larger battery bank available, you can back up your entire house with a system like this. In off-grid projects, this is the only battery backup system type we’ll consider. Battery systems with Hybrid inverters are the most robust, durable solar battery systems on the market today.

Integrated Systems with Battery Backup

The third type of battery system we’ll call an integrated system. This system has the inverter and battery combined in a single piece of equipment. The Tesla battery and Sonnen ECO system are two examples of this type of battery backup. (We are not a Tesla dealer for several reasons, the difficulty of getting the product and the lithium technology is unstable than LiFePO batteries). These systems use a proprietary battery and allow you to maximize solar system sell-back since you are adding the battery backup capability to your existing grid-tied solar system.

Any battery systems added to any grid-tied solar system after the fact is possible. Referred to as AC-Coupled systems, these allow the effective addition of a Hybrid battery system to your existing grid-tied system. Installed hybrid inverters are compatible with existing solar systems that are AC-Coupled. Adding another inverter controls the battery function and allows the solar production inverter to connect to it. However, the integrated system batteries are proprietary, and these systems also limit battery capacity. In addition, because the inverter and battery are integrated, you also have to add inverter capacity to add battery capacity. That limits your system flexibility.

Also, you have double the inverter count because you are adding a Hybrid inverter to your existing system. In AC-Coupled systems, your solar system inverts DC power from the solar panels to AC, and then the Integrated system’s inverter takes the AC power back to DC to store in the batteries. It’s less efficient than the actual Hybrid battery systems because of this.

Conclusion

In summary, if you want your new solar system to maximize sell back to the grid and minimize the time it takes to pay back your initial investment, you need to consider a grid-tied system and its integrated battery. On the other hand, if you want an off-grid system or a more robust, complete backup source for heavy loads, you’ll want to consider the Hybrid systems. In either case, we can design and price a system to meet your needs.

Attached is a comparison chart showing the different solar battery backup system options.