There are a lot of decisions to be made when designing the perfect solar system for yourself: price, power capabilities, warranties, and all the rest. But one decision that looms largest, perhaps, is the choice of an inverter: whether it’s string or microinverters. Specifically, the component that converts the direct current (DC) electricity generated by your solar panels to the alternating current (AC) electricity used in your house.
Initially, there was only one inverter option, a string inverter. With a string inverter, your panels are connected by a “string” of cables to a central current-conversion device. Think of it as Christmas tree lights: the individual lights (in our case, solar panels) are connected via a cord (our “string” cable system) which plugs into the power source (the string inverter). However, two other approaches to the conversion problem have leaped to the forefront of solar innovation, particularly for residential solar. One is the microinverter, in which each panel has its own power inverter tucked under it on the roof. This is an option popularized by a company called Enphase. The second inverter type is the panel by panel “optimizers” approach, in which each panel is tied to a central inverter. This is an option pioneered by a company called SolarEdge.
In fact, the latter two alternatives currently dominate the U.S. residential solar energy market, as people relegated string inverters to a “commercial-size system”-only technology.
Let’s take a step back to review how we got here. Both newer technologies gained their market popularity in two key ways. First, these newer inverter types offered a panel-by-panel optimization capability. This feature (built into either in the microinverter itself or in the optimizer at the panel level) is designed to compensate for uneven shade across the solar array such that when one panel is shaded, but the others are not, the whole system will still perform effectively. Second, microinverters and panel level optimizers made a panel-by-panel full system shutdown possible.
Panel-by-panel shutdown was important and became a new electrical code requirement in 2017 that grew out of firefighters’ concern that a panel could electrocute a firefighter if a panel were still “active” in the case of an emergency. Electrocution, particularly with a DC system, is indeed concerning.
But, I’m here to tell you that string inverters are still very much a viable option, just as viable as Enphase’s microinverter or SolarEdge’s power optimizer inverter, in many residential applications. Particularly if you are looking for the best price, greater reliability, and the most widely used inverter technology in the world.
Here are just a few of the many myths about string inverters and why, in your case, a string inverter may be the way to go.
Myth #1: Microinverters are more reliable.
This one I don’t understand, but I often hear it. In electronic components, the potential for a system failure is generally correlated with the number of individual components. There are simply more potential points for a breakdown. Since a microinverter performs both an optimization process as well as the DC to AC conversion, each and every unit has a lot of components and thus more potential points of failure, especially outside in New England where they are exposed to humidity and huge seasonal temperature fluctuations. That does not bode well for the microinverter. Plus, string inverter technology has been used since solar energy’s earliest days in comparison with the newer technology of a microinverter that has more opportunities for failure.
This is not to say that microinverters are bad and are always going to fail. But rather, there are many cases where you could save both money and avoid product complexity by considering all of your options.
Myth #2: Microinverters are cheap and easy to replace.
Microinverters are typically the more expensive option. Not just upfront, but also in the maintenance they often require. All arguments of reliability aside, we can replace a string inverter in less time and with less manpower than a single microinverter. Yes, when a string inverter fails the entire system fails, but it is certainly more obvious and depending on the weight of the inverter and tools available, a string inverter can be replaced by a single installer without getting on the roof as most string inverters are located at ground level. While with microinverters each panel has a small inverter tucked underneath. So to fix one, we have to go up onto the roof, remove the panel, replace the inverter, and then put it all back. Clearly, that’s not easy and for safety, it requires two people. All in, that single microinverter replacement could cost $1000 or more.
With microinverters traditionally costing more per peak watt and being more complex to install, string inverters are most definitely cheaper and easier to replace.
Myth #3: Putting complex electronics on the roof is the only way to address fire safety.
As I touched upon earlier, the call for microinverters was bolstered by the requirement for panel-by-panel shut down because of the perceived risk of electrocution should a firefighter need to get on the roof. In short, microinverters were the first to have a solution to that problem, followed quickly thereafter by optimizer technology. According to Solar Reviews, “Microinverters comply with these rapid shutdown requirements and have this capability embedded into each module.” However, that has changed as vendors like SMA have figured out how to provide a rapid shutdown capability with far fewer components than microinverters or optimizers.
String inverter manufacturers do this by pairing each panel with MLPE (module-level power electronics). Or in the case of SMA, they have their own solution called SunSpec-certified TS4-R-F created specifically for the rapid shutdown. The net result is thus the same firefighter safety in, a more cost-effective package than a microinverter or optimizer; while as noted also with far fewer electronics (and thus a much lower potential for failure).
Myth #4: Microinverters eliminate the inverter’s “single point of failure.”
Microinverter providers report that of all technical components, the inverter is the component most prone to malfunction. That’s true I believe. So in the case of a string-inverter system, inverter failure causes the whole system to shut down. This is what people call the “single point of failure” scenario because inverter failure leads to total system failure. However, fixing this issue as noted above is relatively simple. If your string inverter from SolarEdge or SMA fails, responding electricians can easily access the failed inverter on the side of your home or in your garage.
Contrastingly, microinverter malfunction triggers only the corresponding panel to shut down rather than the whole system. Although this appears to be quite an advantage, the replacement of the failed microinverter involves more labor on the roof. Since microinverters exist under the solar module, solar electricians must remove and reinstall the affected panel to connect a new microinverter. And since the inverter is the most likely component to fail, microinverters are just replacing the “single” point of failure for the whole system with a “single” point of failure for each panel. For example, a 30-panel-system would now have 30 points of failure, meaning not only will it be more likely to fail, but it’s also more expensive to fix as I said in myths number one and two. And worse, if you only look at your daily electricity production as a guide one or just a few microinverter failures may easily go unnoticed.
So, while many people see “the single point of failure” argument as a disadvantage for string inverters, I see it as one for microinverters too.
Myth #5: A shaded panel will bring down the output of every other panel in a series string with a string inverter.
The major push for microinverter technology was to address the issue of shading. When your home or business has several trees or obstructions blocking direct sunlight, it is said by Enphase that the shade on one panel could bring down the output of any panels wired in the series. This means when one panel is producing poorly, all the panel production levels will drop to that lowest performing panel. Therefore, any shading means a string inverter might not be the best choice. I’m here to say this is untrue.
While it’s true back in the day that string inverters may not have handled the drop out of a panel well, this was largely resolved when string inverters started adding multiple MPPT inputs. For those who don’t know, MPPT stands for Maximum Power Point Tracker. According to Solar Choice, it’s essentially a circuit, typically a DC to DC converter, that functions to maximize the energy available from the connected solar module arrays at any time during its operation. This is important because, without an MPPT circuit, the inverter would not be operating optimally. So, as the MPPT circuit constantly monitors the array’s voltage and current, it attempts to drive the operating point of the inverter to the maximum power point of the array. This squeezes the most usable energy possible out of a string of solar panels. All it takes is adjusting the voltage to always give the inverter the most suitable input range.
Most SMA inverters have 3 MPPT inputs (although the 3.0 and 3.8 inverters have only 2). So, if you had a thirty-panel system and one panel is completely shaded, it wouldn’t affect all 30 panels. Instead, the inverter MPPT does a sweep of the IV curve to see if the power of the string inverter would increase if it bypassed the shaded panels. This sweep function is unique to SMA and what they call their Shadefix. Once that technology is used, if you had 3 strings of 10, it would only affect one individual string.
Many modern panels also come with devices called bypass diodes. According to CED Green Tech, these minimize the effects of partial shading by enabling electricity to flow around the shaded cells. When a module becomes shaded, the bypass diode begins conducting a current through itself, so the current greater than the shaded cell’s new short circuit current is “bypassed” through the diode. This decreases the amount of heating at the shaded area making the module function more normally. In simpler terms, these bypass diodes allow electricity to “bypass” shaded cells in the modules and take a different path through the cells of the module. Similar to moving traffic, if one street is closed the cars will just have to take another route.
All electrical jargon aside, these two technologies allow string inverters to function just as well as microinverters despite shading or low production on one panel. That said, at the end of the day you should let your solar consultant help you decide. By looking at the specifics of your home, we’ll be able to best serve your inverter needs. It’s just important to remember that string inverters, despite being an older technology, are easier to maintain, cheaper to install, and still function just as well with the use of MPPT and bypass diodes in a majority of cases and should therefore always be considered when you start planning a new system!