Choosing a Battery-Based PV Inverter

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.

Generator Support

Many off-grid inverters can operate in parallel with a generator, instead of just switching the loads to generator power when the generator comes on. This allows an inverter to “assist” a small generator with large loads. Historically, generators were sized to simultaneously power the largest loads and charge the batteries. Now, with greater inverter capacity, the inverter may be sized to serve the largest loads, with a small generator sized to handle only battery charging. The inverters that can operate in parallel with a generator (often called “generator support”) can help a smaller generator start a large load like a well pump or table saw by briefly drawing power from the batteries.

AC Output Needs

Some inverters provide only 120 VAC output; if your off-grid house (or the critical loads subpanel in your battery backup system) requires 240 volts, a second inverter is added to provide the other phase. Instead of adding a second inverter, an external step-up transformer can be used to get 240 VAC from a 120 VAC inverter. The efficiency is reduced, but the cost may be quite a bit lower than adding a second inverter. Some inverters come with 120/240 VAC split-phase output. Which is best depends on your situation.

If you have no 240 VAC loads, you can use a single 120 VAC inverter to energize both 120 VAC legs of your load panel (see the “Beware: Multiwire Branch Circuits” sidebar). If you have an appliance that requires 240 VAC, such as an existing well pump, you have a couple of choices:

• If you need 240 VAC, but don’t need the combined power of two inverters, then it can make sense to get a single 120/240 VAC split-phase inverter. For example, if you have a 1 hp deep well pump that draws 2,000 W with a 7,000 W surge at 240 VAC, you could save money by using a single 4,000 W 120/240 VAC inverter to power it, rather than two 3,600 W 120 VAC inverters. On the flipside, these split-phase inverters won’t put out full power on a single leg—they are usually limited to about 67% or 75% of full power on a single leg. So if you have a very large 120 VAC load, a single 120 VAC inverter may be better than a split-phase inverter of the same rating. For example, a 4,000 W, 120 VAC load could not be powered by a 4,000 W, split-phase inverter.
• If you need 240 VAC and the combined power of two inverters, there are two options. One is to use two 120 VAC inverters stacked in series, and the other is to use two 120/240 VAC inverters stacked in parallel. Using two 120/240 VAC inverters gives redundancy—if one fails, you can still get 240 VAC from the other inverter. This method can also be more efficient, because, for small wattage loads, only one inverter needs to be on. Sometimes the choice depends on the model. Some, notably SMA America’s Sunny Island series and OutBack Power’s line of FX inverters, only come in 120 VAC, so you will be selecting one inverter per phase when using multiple inverters.

A 120/240 VAC inverter is often selected for a battery-backup grid-tied system because it’s cheaper and easier to install. The amperage of the tie-in is half as much at 240 V compared to the tie-in at 120 V. This means you can fit twice as much PV power on a given service size following NEC 705.12(D), which commonly limits the size of the solar input to 20% of the busbar amperage.

Balance of System

Remember that an inverter is only one part of the system—many people focus on selecting and buying the inverter, and then face the challenge of integrating it with rest of the equipment. Magnum Energy, OutBack Power, and Schneider Electric offer wiring solutions (aka “power centers” or “power panels”) for use with their inverters, simplify the wiring considerably. There are also third-party options, such as MidNite Solar’s E-Panel, which provide complete Code-compliant wiring systems to simplify an inverter’s installation. Most inverters from Magnum Energy, OutBack Power, and Schneider Electric require a separate system control panel for programming and monitoring. There are no controls or displays on the inverter itself. This can be good when the inverter is located in a utility room, but, for example, you also want an inverter control/monitor in the living room. This functionality comes at an extra cost—between $150 and $400 depending on the model.

Many of the advanced functions, such as automatic generator-start, are part of the system control panel, not the inverter firmware—without the control panel, you may be limited to just turning the inverter on and off, and not be able to adjust the settings.

Many battery-based inverters can connect to a computer for remote monitoring, control, and data logging. Some allow users to remotely monitor the inverter’s operation via the Web. This usually requires an extra communications box (which may or may not be the same as the remote system control panel).