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The most visible element of a rooftop solar system is, obviously, the panels. And, where I live in New England, I’m seeing more and more of those glossy- or matte-black units topping the homes around me, helping to power more of the local load with green energy while also pushing down their owners’ monthly utility bill.

Solar Inverters: Bringing (Green) Power to the People

Chuck Ross
The most visible element of a rooftop solar system is, obviously, the panels. And, where I live in New England, I’m seeing more and more of those glossy- or matte-black units topping the homes around me, helping to power more of the local load with green energy while also pushing down their owners’ monthly utility bill. But from an electrical viewpoint, the more interesting work of putting solar-generated electrons to work occurs out of sight, in the inverters that convert the direct-current (DC) electricity panels produce, into alternating current (AC), for use in the home and the connected grid. Homeowners have some choices when it comes to types of inverters, and likely will be looking to their electrical contractor for advice. Here's some information to help customers make informed decisions.

What’s available
Today’s inverter offerings fall into three major categories: string inverters, string inverters with power optimizers and micro inverters. Each comes with benefits and tradeoffs.

  • String inverters are the simplest and least expensive approach. These are centralized units serving an entire array (or portion of an array for larger systems). Panels are wired to each other in series, and then to the inverter, where the AC conversion takes place. This is a relatively simple process and the least expensive to install. The major drawback, though, is that a single shaded or malfunctioning panel can drag down – or shut down – power output of the entire array. So, if a roof features a mix of all-day sun and partial shade, microinverters or power optimizers might be a better choice.
  • Microinverters are, as the term implies, much smaller units installed on the underside of each panel. These devices are each connected in parallel to a trunk wire that leads to a dedicated breaker box. This design maximizes a system’s output because an individual panels can’t affect the performance of the others in the system. Plus, owners can monitor the output of individual panels, not just the entire system, making troubleshooting easier if power production suddenly drops. This is also a more expensive approach, however, because of the added wiring (although some panel makers offer products with microinverters already installed). It also creates more possible points of failure (though, again, at least a failure in a single panel won’t bring the rest of the system down), so warranties are especially important with these products.
  • Power optimizers are a newer class of product that combine benefits of string inverters and microinverters. They’re mounted to each panel like a microinverter, but they only “optimize,” or smooth out, the panel voltage. Conversion from DC to AC still takes place in a centralized string inverter. So, as with microinverters, panels each can contribute their maximum output, whether they’re in partial shade or full sunlight. And they also allow for individual panel monitoring. However, there’s still the challenge that a failure of the central inverter will bring the whole system down. Inverter warranties typically top out at 10 years, while both microinverters and power optimizers can last up to 25 years, so this isn’t an unlikely possibility. Cost-wise, power optimizers fall between the other two approaches.

Decisions, decisions
It’s great contractors now have options to address multiple rooftop configurations. If a roof is ideally sited, with no nearby trees, a string inverter could be the most affordable choice (though a buyer might still want the monitoring and troubleshooting capabilities the other two technologies provide). Microinverters and power optimizers can be better choices where partial or occasional shading is involved, however choosing one of these approaches over the other is more nuanced. Both offer monitoring and troubleshooting, but power optimizers might be more exposed to future issues related to a string inverter failure, which could lead some homeowners to opt for more expensive microinverters.
Photo courtesy of Mersen
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