Choosing a Battery-Based PV Inverter

Intermediate

Inside this Article

Magnum Energy (magnumenergy.com)
Magnum Energy makes a variety of inverters for off-grid and mobile applications, including options like remote monitoring, battery monitoring, and automatic generator start.
OutBack Power’s FLEXware
Several manufacturers produce balance-of-system components that integrate gracefully with their inverters to make installation clean and easy. OutBack Power’s FLEXware (shown here) includes metering, AC and DC overcurrent protection and disconnects, and charge controllers.
OutBack Power (outbackpower.com)
OutBack Power makes two lines of inverters for use in both grid-tied and off-grid systems. Its Radian line features advanced grid interaction and battery support. Its FX line is newly expanded with more size options and rugged housings for installation in extreme climates.
Schneider Electric (sesolar.com)
Schneider Electric has expanded its XW line of grid-interactive inverters and added new advanced grid features and Internet monitoring. Its SW line of off-grid inverters has been expanded as well.
SMA America (sma-america.com)
SMA America’s Sunny Island inverters are optimized to work in AC-coupled systems with its grid-tied inverters. System monitoring can now be accomplished via a smartphone.
MidNite Solar's packaged balance-of-system components
Third-party manufacturers like MidNite Solar also produce packaged balance-of-system components designed to integrate with the major inverter brands.
Magnum Energy (magnumenergy.com)
OutBack Power’s FLEXware
OutBack Power (outbackpower.com)
Schneider Electric (sesolar.com)
SMA America (sma-america.com)
MidNite Solar's packaged balance-of-system components

The inverter is the heart of a battery-based PV system, converting DC from the batteries into AC for lights and appliances. High-power options, better surge capacity, lower cost per watt, and more bells and whistles are now available.

Matching the Inverter to the System

There are basically two different system configurations that utilize battery-based inverters: “off-grid” (also referred to as “stand-alone”) and those that have utility power available. Within the two system types are numerous variations. Determining which inverter is appropriate for your system requires answering several questions:

  • Is there is access to a supplemental power source, such as the grid or a generator?
  • What are the goals for your system? If you’re planning an off-grid system, do you want to minimize generator size? If you are on the grid, do you want to maximize the solar power that’s exported to the grid? Or do you want to maximize your on-site consumption of energy produced by your system?

There are myriad possibilities. Some inverters are built to serve only one or two system configurations, while others can accommodate several different system types­—and selecting how the system functions can be as simple as a quick programming change. The basic battery-based system configurations are discussed below. However, selecting the best inverter for your system requires spending some focused time with inverter cut sheets and manuals, and/or working with an installer who has solid experience with battery-based systems.

Off-grid. As the name suggests, these systems do not have access to utility power. Off-grid homes commonly use a generator for supplemental power for large AC loads or during times of little sun. These systems require an inverter/charger that can operate in off-grid mode and can use outside AC input from the generator for charging the battery bank. Several battery-charging inverters have expanded programming options, optimizing the working relationship between the generator and the inverter. As a result, the generator capacity needed can be reduced (see “Generator Support”).

Grid available. If there is utility power available, you can design a grid-tied system where excess energy is sold back to the grid, but a battery bank is available for backup (aka “grid-tied with battery backup”). These systems require an inverter that has a grid-interactive mode, but can be configured several different ways. The most common method is to have the inverter operate in parallel with the grid when it is available, and to provide backup power to specific AC loads when the grid goes down. This minimizes battery use, since it only draws from them if the grid is down.

There are also newer options for systems—“grid support” and “grid zero.” These are programming modes for some grid-interactive inverters that allow you to fine-tune how your system interacts with the utility. These options can be useful in areas where rules and incentives for grid-interactive systems have changed, such as not allowing exporting of PV energy to the grid or not allowing net metering, making consuming energy from the on-site solar and battery bank more desirable. Some inverters can also accommodate a second AC power source, such as a generator, to provide another level of backup power.

Alternatively, an inverter/battery system can function as a backup system to the grid (i.e., a UPS system) or can use the grid as a backup power source to a solar/inverter/battery system—without exporting any energy to the grid. These systems require inverters that can accept AC power from the grid for battery charging, but do not have to be listed as “grid-interactive.”

Surge Capacity

Inverter surge is a measure of how much power the inverter can put out to start motor loads that may draw much higher than normal power upon startup. Depending on the particular motor, this may take from less than a second to tens of seconds, and may be from 1.5 times to 7 times the motor’s normal load. There is no standard in rating inverter surge capacity, so what one inverter reports as “surge” may not directly compare to another one. A “surge duration” is more useful information than a generic “surge” rating with no specs on duration. One way to determine how an inverter handles surge is reflected in its weight—heavier transformer-based inverters can sustain a good surge for much longer (minutes versus seconds) than a lighter-weight high-frequency inverter. This is one large difference between the inverters designed for whole-house use included in this article compared to many RV and consumer-electronics inverters.

Comments (2)

danyboy's picture

Learning so much about what's available out there before I buy a pv system soon.
This website is a goldmine of information.

Brandon Williams's picture

This is a great article, Zeke! What is your favorite new inverter to work with? I am happy to see that inverter companies are offering new features to support new types of batteries that are coming out (nickel iron, lithium iron phosphate, etc.)

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