How to choose between string and microinverters in residential solar projects

Solar Power World Article

By Kelsey Misbrener | August 12, 2019

 A New Day Solar installer secures a microinverter to racking.
A New Day Solar installer secures a microinverter to racking.

String inverters and microinverters take very different approaches to converting DC power to usable AC power in solar projects. In a string inverter installation, high-voltage DC power from all panels in an array is routed into one inverter where it’s converted to AC power. In a microinverter system, a small inverter is connected to each module of an array (with the exception of new four-in-one models from APsystems), DC power is converted to AC power at the module level, then routed into the home for use.

Solar Power World polled the 2019 class of Top Solar Contractors on which inverter type is preferred on residential systems, and respondents were nearly split down the middle with string inverters just pulling ahead. But the random residential contractors interviewed for this story mostly chose microinverters. The two technologies offer distinct advantages and weak points.

Installation process

Standalone string inverters can be installed at ground-level, typically in a customer’s garage. However, advancing technology and rapid shutdown requirements mean most string inverters now need additional components. To be rapid shutdown code compliant, string inverters must be paired with module-level power electronics (MLPE) capable of shutting down the array in case of emergency. When a contractor has to install both a string inverter and an optimizer on the panel, the benefit of a string inverter’s quick install is negated.

“Recent electrical code changes have made this technology more complicated and costly than before due to the lack of built-in rapid shutdown functionality,” said Scott Carlson, president of California installer New Day Solar.

Still, many optimizers come already embedded to the modules (in smart module solutions) or can be pre-attached on the panels by installers before they’re brought up to the roof.

Some microinverters can also be pre-installed right on the module (or come pre-installed like in AC module solutions)but others must be installed on the solar racking. Securing them while on the roof adds a level of difficulty.

Microinverter installation is made easier since no additional products are needed to meet rapid shutdown compliance. That capability is built into the module-level solution — for example, when the AC circuit is flipped, APsystems’ microinverters shut down in 100 milliseconds on their own.

“By its nature it’s already compliant,” said Jason Higginson, senior director of marketing at APsystems.

Since microinverters are converting the power to AC on the roof, installers don’t have to apply DC power safety labels or install DC disconnects. These safety components are absolutely required when installing string inverters, said Brandon Carlson, VP of New Day Solar.

Operations & maintenance

String inverters are typically mounted to a wall in a garage, protected from the elements. Although microinverters are housed underneath solar panels, they’re still exposed in a way string inverters aren’t.

“With the microinverter, you’re taking all of the technology that typically resides in an inverter and you’re putting it in the most hostile environment possible, which is right on the roof underneath the module,” said Brad Dore, director of marketing for string inverter manufacturer SMA. “So it has to perform at a higher level in terms of reliability to equal that of a string inverter system.”

In addition to environmental wear and tear, inverters are the most hardworking piece of a solar project and thus the most likely to fail. When it comes time to repair or replace a string inverter, it’s as easy as walking into the garage to troubleshoot or replace one unit. With microinverters, installers must get back on the roof and remove panels to get to the units.

“I always prefer to deal with a string inverter simply because it’s on the ground. Anytime you’re climbing on the roof, you’re packing and unpacking the truck to get the ladders out, fall rescue is needed and so on. It’s just a lot more involved,” said Bryan Whitton, product manager at Darfon, which sells hybrid inverters for the U.S. market and microinverters in other countries.

However, Whitton said microinverters have become very reliable and rarely require maintenance. And O&M savings on the ground for string inverters is negated when the paired MLPE on the roof go bad, requiring roof service.

Brandon with New Day Solar said that although power optimizers and microinverters both typically come with 25-year warranties, string inverter warranties remain at about half that length. So even when MLPE are added to string systems, the overall system isn’t covered as long as a micro system. Brandon said microinverters’ higher cost is well worth it considering their reliability and long warranty.

“As someone who has used both technologies frequently, the failure rates for both are rather small and the cost [isn’t] significant, especially when one [considers an] optimizer system needing an inverter replacement sooner than a microinverter system, but not covered under the same length of warranty,” Brandon said.

San Diego installer Sunline Energy also prefers microinverters for their reliability.

“We prefer microinverters for the simple fact that they’re proven to be more reliable than string inverters,” said Gerry Ebert, Sunline Energy’s director of sales and marketing. “This has led to minimizing production losses for our customers and reduced truck rolls for our service department.”

Even if errors do occur in microinverter systems, the module-level inverters make it easy to monitor and pinpoint the failed inverter, said Eric Hansen, principal owner and general manager of Oregon’s True South Solar. With string inverters, more time would be needed to find the source of the problem.

Power production

With microinverter systems, if one solar panel is compromised, the entire array’s power production won’t be impacted. With a string inverter alone, a system is only as good as its least productive panel.

“Because inverters are tied to each panel, inefficiencies within a given panel do not drag down the rest of the system, thereby increasing the resiliency of the system’s efficiency,” said True South Solar’s Hansen. “For residential systems, microinverters allow the maximum amount of viable space to be used and allow ease of system expansion at a later date if the customer so chooses.”

When a power optimizer is added to a string inverter configuration, installers can likewise ensure that each panel is independently producing power and the whole system isn’t brought down by one shaded panel. Depending on an installer’s choice, MLPE paired with string inverters can help with shade mitigation, rapid shutdown, powerline communication and monitoring. This flexibility in optimizer choice is another bonus with string inverter installations.

“It offers more design flexibility because an installer can choose the model that works for their use case, and it provides, ultimately, a solution that scales better,” Dore said. “Whether somebody needs it for a small system, a medium system, a large system, whether they need it to address multiple roof orientations or shading, we’ve got a system that’s really ideal for that use-case.”

North Carolina solar installer Yes Solar Solutions prefers to use SolarEdge inverters combined with optimizers for their flexibility.

“Optimizers help mitigate shading of solar modules on strings, as well as boost production in all types of weather conditions,” said Kathy Miller, co-founder and CEO of Yes Solar Solutions. “SolarEdge also offers a great solution for the module-level rapid shutdown requirement and both solar production and consumption monitoring, which is a very attractive feature for homeowners.”

As the technology continues to evolve, installers should stay apprised of the latest string and microinverter models for residential installations to give their customers the best choice for their specific installation.

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