Selecting a Solar Inverter

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Inverters are an indispensable component of almost all solar electric systems. They work long hours converting the direct current (DC) electricity generated by a photovoltaic (PV) array into alternating current (AC) electricity—the type used in homes and businesses. In battery-based systems, inverters contain circuitry to perform a number of additional useful functions.

In this article, I’ll discuss the three types of inverters and the features you should look for when purchasing one. But first, do you actually need an inverter? 

A good inverter is key to the success of most solar electric systems.
Photo by Adobe Stock/DIY Ana Dimitrova

Do You Need an Inverter?

Although this may seem like a ridiculous question, some solar applications operate solely on DC power and, as a result, don’t require inverters. Included in this category are small solar electric systems that power a few DC circuits in rustic, out-of-the-way cabins and cottages; recreational vehicles; and sailboats. It also includes direct water-pumping systems that produce DC electricity to power DC water pumps—for livestock, for example. My DC solar pond aerator and the DC fans in my Chinese greenhouse are powered directly by PV modules. But virtually all other renewable energy systems require an inverter. The type of inverter needed depends on the type of system.

Inverters come in many shapes, sizes, and prices. The smallest inverters, referred to as “pocket inverters,” range from 50 to 200 watts. They’re ideal for supplying small loads, such as computers, radios, and televisions. Most homes and small businesses, however, require inverters in the 2,500-to-12,000-watt range. So, which inverter should you select?

Types of Inverters

Inverters come in three basic types: those designed for battery-less grid-connected systems; those designed solely for off-grid systems; and those made for grid-connected systems with battery backup. “PV System and Inverter Terminology” below lists the alternative names for inverters and PV systems that you may encounter.

If you’re going to have a system installed by a professional, your installer will specify the inverter. Installers usually purchase solar equipment from local distributors, which, in turn, typically carry inverters from one or two manufacturers. Your installer will likely choose their favorite inverter.

If you’re installing your own solar setup, you’ll need to select a reliable inverter that matches your system. Designing and installing a PV system can be quite challenging; there’s a lot to know. It’s a task best undertaken by those with a good working knowledge of electrical wiring and solar electric systems. If you decide to go the DIY route, consider taking a few hands-on PV installation workshops beforehand.

If placing an inverter outdoors, locate it in a spot that receives plenty of shade throughout the day.
Photo by Adobe Stock/Varga

You can purchase solar kits online from a number of companies, including Northern Arizona Wind & Sun. Kits include all the components you’ll need to install a system, and they should include wiring diagrams as well. While a kit is a great option, be sure to check out the equipment, especially the inverter. Find out where it’s manufactured and the details of its warranty. Before you purchase a kit, confirm where you’ll be able to get the equipment serviced. You don’t want to be stuck with an inverter that can only be serviced outside of your area. To avoid this issue, buy kits that come with equipment from major solar manufacturers with nearby service centers.

The first consideration when shopping for an inverter is the type of system you’re installing. If you’re installing a grid-connected system without batteries, you’ll need an inverter designed for such systems. If you’re installing an off-grid system, you’ll need an off-grid inverter charger. If you’re installing a grid-connected system with battery backup, you must purchase an inverter that’s both grid- and battery-compatible—a multimode inverter. Multimode inverters may also be a good choice for off-grid systems, in case you decide to connect to the utility at a later date.

Local or online suppliers can help you choose the right inverter for your setup; however, the more you know, the more their advice will make sense.

Grid-Tied Inverters

Shopping for a grid-tied inverter is relatively easy. You’ll need an inverter that matches the size of your solar array. For example, if you need a 5-kW array to meet your electrical needs, you’ll need a 5-kW battery-less grid-tied inverter. (This inverter will service systems from 4.8 to 5.2 kW.)

After that, most people shop for efficiency. Modern grid-tied inverters tend to be around 96 percent efficient. Add optimizers, and you’ll boost the efficiency a bit more. Install microinverters rather than a string inverter, and you’ll also enjoy a slightly higher efficiency.

Be sure the inverter is UL 1741 listed. This assures the utility that your inverter has anti-islanding capability. Also, be sure to check out the warranty. Most inverters come with a 5-year warranty, although 10-year warranties are becoming more common. In some instances, you may be able to purchase an extended warranty for a string inverter. I’d strongly recommend it. Microinverters and optimizers come with 25-year warranties. I only install microinverters.

Check the output voltage of the inverter. Homes are typically wired at 240 volts, so you’ll probably need a 240-volt inverter. Commercial buildings are typically wired at 208, 480, or 600 volts. Be sure your inverter matches the voltage of your application.

When shopping for an inverter, study the DC voltage input range so you can determine the length (number of modules) of series strings you can wire into that inverter. The number of modules affects the voltage of each series string. Most string inverters can be wired to series strings of 10 to 12 modules.

Inverters come in many shapes, sizes, and prices.
Photo by Adobe Stock/ZStock

Battery-Based Inverters

If you’re going off-grid or installing a grid-tied system with battery backup, you’ll find that choosing an inverter requires a little more know-how. In this section, I’ll discuss the key factors that go into choosing a battery-based inverter, including input voltage, waveform, continuous output, surge capacity, and efficiency.

Input voltage. When shopping for a battery-based inverter, you’ll need to select one with an input voltage that corresponds with the voltage of your battery bank. (That’s because, in these systems, batteries feed DC electricity to inverters.)

Battery banks in solar electric systems are typically wired at either 12, 24, or 48 volts. Twelve-volt systems are common in cabins and summer cottages. Residential systems are either 24- or 48- volt systems; most commonly 48 volts, because those systems are the most efficient. Be sure the input voltage of the inverter you choose matches your battery bank’s voltage.

Waveform: modified square wave versus sine wave. Waveform is a rather technical topic. It’s a measure pertaining to the voltage of AC electricity. Unfortunately, it’s beyond the scope of this article. Remember this, though: Battery-based inverters for off-grid systems are available in either modified square wave (often called “modified sine wave”) or sine wave. Because the wave form of electricity on the electrical grid is sine wave, all grid-connected inverters produce sine wave AC electricity. That way, their output matches utility power. Off-grid inverters can be either sine wave or modified square wave. Multimode inverters for grid-tied systems with battery backup must be sine wave.

I strongly recommend sine wave inverters no matter your application, even if going off-grid. Modified square wave electricity is a crude approximation of grid power. It works well in many appliances, including refrigerators and older washing machines, as well as power tools. It also works pretty well in most electrical devices, including TVs, lights, stereos, computers, and inkjet printers. Although all these devices can operate on this lower quality waveform, they run less efficiently, producing more heat and less work for a given input.

Bigger problems arise, however, when modified square wave is fed into sensitive electronic circuitry, such as microprocessor-controlled front-loading washing machines, appliances with digital clocks, chargers for various cordless tools, copiers, computers, and laser printers. These devices require sine wave electricity. Without it, you’re sunk. In my off-grid home in Colorado, for example, I found that my energy-efficient front-loading washing machine wouldn’t run on the modified square wave electricity produced by my inverter. The microprocessor that controls the washing machine simply can’t operate on this inferior form of electricity. After I replaced the inverter with a sine wave inverter, the washing machine ran fine.

Making matters worse, I found that some electronic equipment, such as TVs and stereos, gave off an annoying high-pitched hum when operating on modified square wave electricity. Modified square wave electricity may also produce annoying lines on TVs, and can even damage sensitive electronic equipment.

When operated on modified square wave electricity, microwaves cook slower. Equipment and appliances also run warmer and might give out sooner, and computers and other digital devices have more errors and crashes. (The only time I’ve burned out a computer was when I was powering my off-grid home with a modified square wave inverter.) In addition to these problems, digital clocks don’t maintain their settings as well when operating on modified square wave electricity, and motors don’t always operate at their intended speeds.

So, given all the complications, why do manufacturers produce modified square wave inverters? The most important reason is cost. Modified square wave inverters are much cheaper than sine wave inverters. You’ll likely pay 30 to 50 percent less for one. Modified square wave inverters are also durable. They work hard for many years with very little, if any, maintenance requirements (Their durability may be related to their simplicity: They’re electronically less complex than sine wave inverters.) Regardless, battery-based sine wave inverters for off-grid systems are typically a better choice. Their output is well-suited for use in modern homes with sensitive electronic equipment. SMA, Schneider Electric, and OutBack Power all produce excellent battery-based sine wave inverters.

Continuous output. Continuous output is a measure of the power a battery-based inverter can produce on a continuous basis — provided there’s enough energy available in the system. The power output of an inverter is measured in watts, although some inverter spec sheets also list continuous output in amps. (To convert watts to amps, use the formula “watts equals amps multiplied by volts.”) Schneider Electric’s (formerly Xantrex) sine wave inverter, model SW2524, for instance, is capable of producing 2,500 watts of continuous power. This inverter can power a microwave using 1,000 watts, an electric hair dryer using 1,200 watts, and several smaller loads simultaneously without a hitch. (The “25” in the model number indicates the unit’s continuous power output; it stands for 2,500 watts. The “24” indicates that this model is designed for a 24-volt PV system.) The spec sheet on this inverter lists the continuous output as 21 amps.

OutBack’s sine wave inverter VFX3524 produces 3,500 watts of continuous power and is designed for use in 24-volt systems. Off-grid homes can easily get by on a 3,000-to-4,000-watt inverter.

To determine how much continuous output you’ll need, add up the wattages of the common appliances you think will be operating at once. Be reasonable, though. Typically, only two or three large loads operate simultaneously. A washer and well pump may be operating when a sump pump is running, for example. If you’re planning to operate a shop next to your home on the same inverter, you’ll need an inverter with a higher continuous output, or you’ll need to install a generator.

Surge capacity. Electrical devices with motors, such as vacuum cleaners, refrigerators, washing machines, and power tools, require a surge of power to start up. It’s a momentary spike in amperage — typically lasting only a fraction of a second — that’s required to get the motor running. If an inverter doesn’t provide a sufficient amount of power, the power tool or appliance won’t start, which is more than just inconvenient. A motor that’s not receiving enough power to start will draw excessive current and overheat quickly. Unless the motor is protected with a thermal cutout, it may burn out.

When shopping for an inverter, be sure to check out surge capacities. All quality solar inverters are designed to permit a large surge of power over a short period of time, usually about five seconds. Surge capacity or surge power is listed on spec sheets in either watts or amps.

Efficiency. Inverter efficiency is calculated by dividing the energy coming out of an inverter by the energy going in, and then multiplying by 100 to get a percentage.

Most battery-based inverters have efficiencies in the low-to-mid-90s. Although there’s nothing you can do about it, you should note that the efficiency of inverters varies with load — how much power is being used. Generally, an inverter achieves its highest efficiency once output reaches 20 to 30 percent of its rated capacity. A 4,000-watt inverter, for instance, will be most efficient at outputs above 800 to 1,200 watts. At lower outputs, efficiency is dramatically reduced.

Conclusion

A good inverter is key to the success of a renewable energy system, so shop carefully. Size it appropriately, and be sure to consider future electrical needs. But don’t forget that you can trim electrical consumption by installing energy-efficient electronic devices and appliances. Maximizing efficiency is always cheaper than adding more capacity. When shopping for an inverter, select the features you want, and purchase the best model you can afford. Although modified square wave inverters work for most applications, you’ll likely be happier if you purchase a sine wave inverter. It’s a decision you won’t regret if you plan to operate a computer or television.


PV Array Voltage and String Size

To help installers determine how many modules they can wire in a string, virtually all inverter manufacturers provide online calculators. To calculate string size, simply enter the temperature conditions of the site (you’ll need to provide low-temperature data) and the type of module you’re going to install. The online calculator will provide the maximum, minimum, and ideal number of modules in a series string (string length) for each of a manufacturer’s inverters.

Temperature is important when determining the size of series strings in a PV array, because the output of a PV array changes with temperature. Low temperatures, for example, increase the voltage of an array. If an array hasn’t been sized carefully, the voltage of the incoming DC electricity could exceed the rated capacity of the inverter, causing serious damage.

High temperatures, in contrast, reduce the output of an array. If the voltage of a PV array falls below the rated input of a grid-tied inverter, a PV system will shut down and remain off until the array cools down and the voltage increases. This, of course, reduces the efficiency of a system.

[Chart] PV System and Inverter Terminology


Dan Chiras is an internationally acclaimed expert in green homes and renewable energy. This is excerpted from his book Solar Electricity Basics (New Society Publishers).


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