Battery-Based PV System Expansion

Inverters for Battery-Based Systems. Residential battery-based inverters are often large enough to handle the total power of all loads that would operate simultaneously, and still have additional capacity. 

However, if significant loads are added, or if additional battery charging via a generator is required, then more inverter capacity will be needed. This can be planned for by installing a system where multiple inverters can be “stacked”—either in parallel to increase capacity at the same voltage, or to create a 120/240 VAC system if the original system was 120 VAC only. Using AC and DC disconnect boxes with room for additional inverter breakers will facilitate this future upgrade.

It is difficult to mix old and new inverters. Replacing the older inverter may be the best option. Modern inverters are more efficient and have more features, but also may involve significant rewiring of the balance-of-system components.

Higher-voltage charge controllers that can step down voltage can provide more power at the same amperage as nominal voltage charge controllers.

Batteries. Adding new batteries to an old bank is not a good idea, as the newer ones will perform at the level of the oldest ones—think of it as paying new prices for an old battery. Frequently replacing one leads to another, then another, and soon enough it would’ve been a better idea to replace the whole bank. Also, old battery interconnects and leads to the inverter may be corroded and need either cleaning or replacement. If a larger inverter is installed, larger conductors may need to be installed between the battery and inverter.

An old adage in the off-grid market is to not buy the Cadillac of batteries if you’re just learning to drive. While it may take more, smaller batteries to make an equivalently sized bank, and they may not last as long as larger batteries, the price per amp-hour will be less. Any battery, regardless of size or quality, is subject to the effects of abuse, so “learning to drive” on a less expensive bank—which can still provide years and years of reliable service—makes sense. When the batteries eventually need to be replaced, higher-capacity batteries can be installed. If loads have also grown, you may need a larger battery bank. A cost/benefit analysis should be performed to determine which battery is best, including a realistic assessment of the likelihood of regular maintenance.

Charge controllers. Charge controller choices have increased dramatically, with numerous options for expanding old systems. 

In many older battery-based systems, the array’s nominal voltage matches that of the battery bank. Many older controllers are able to operate at 12, 24, or 48 VDC, but this would also require a change in the battery bank and inverter. 

Increasing the DC system voltage, say from 24 to 48 V, means that existing conductors can carry twice or more the power. Rewiring the array for a higher voltage (150 V is common) and using an MPPT controller with voltage step-down means that existing conductors can carry more current—and thus more power, leaving the battery bank and inverter at the original nominal voltage.  

Another option is to add array power with a new controller. This adds redundancy, as long as the old unit still works well. 

Initially sizing for future expansion by installing a larger charge controller means array size can be increased in the future by adding parallel strings. High-voltage charge controllers may allow modules to be added in series to the existing array. 

Power panels designed for stacked inverters and multiple charge controllers don’t need to be populated all at once, allowing easy future expansion.

The redundancy provided by having more than one charge controller increases system reliability—but you’ll definitely need to plan ahead for this. It’s important to consider the conductors required for a second charge controller (or more) when initially installing the system. Installing a DC disconnect box that can hold additional array and controller breakers is also a smart idea.