Choosing a Battery-Based PV Inverter: Page 2 of 2

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

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).

Comments (3)

Edward-Dijeau's picture

Remember that old Uninteruptable Power supply (UPS) that the internal battery died, so, you stuck it on the shelf and purchased a new one or puchased a tablet or laptop that does not need one? Well with a Remote wall switch, for the power cord, and 2-#4 AWG copper conductors bringing your Solar Battery power to the two terminals the old UPS battery pluged into along with a 30 amp automotive 32 volt fuze, you can make it run on grid or off grid with the push of a remote hand held controler. These UPS units were designed and insurred to run your most delicate of electronics so you can bet they will run light bulbs too. UPS units with "Cold Start" feature connected to your batteries provide a clean, quiet power alternative to noisy Fan inverters and can be backed up by utility power that will also send a trickle charge to your batteries on cloudy days or long nights. They come in either 12 volt or 24 volt configurations with the 24 volt versions going as hight as 1500 MA or 800 watts. Since I use a lot of 12 volt lighting, i opted for the 12 volt UPS Units. UPS Units were designed to run on battery backup at Full power only for 10 minuets but can run at up to 60% rated power indefinatly. With the lower wattage LED Lamps and LED Screens of monoitors and TVs, the 500MA to 750MA units run just fine in my home, one in each room for lighting and one for each computer or appliance knowing that with the push of a button, I can safely go back on the grid if needed.

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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