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.
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:
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.”
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.