You’re sitting at your kitchen table, coffee in hand, talking with your solar energy consultant about your future solar installation and production numbers. Then comes the mic-drop, the record scratch.
“For most projects, we use [Brand], which are string inverters. They’re reliable, cost-effective, and work great for most homes.”
Huh?
As a homeowner, you’re not only navigating the new brand names listed on your proposal, but you’re also having to wade through new technology functions like string inverters and microinverters. While the types of inverters serve the same purpose, they achieve it in different ways and have unique advantages and disadvantages.
In the following paragraphs, we’ll explore the key differences between microinverters and string inverters to help homeowners decide what might be the best solution for them.
Table of Contents
What is an Inverter, and Why is it Needed?
Inverters are an essential part of every solar energy system, so much so that solar installers often refer to inverters as the brains of the system. While the solar panels are the most well-known part and produce the energy, that’s where their involvement stops. Inverters are required in almost all scenarios involving power, because solar panels only produce energy in DC, and batteries only store energy in DC.
Inverters take the produced direct current (DC) energy, convert it into useful alternating current (AC) energy, and feed that energy into the home, battery, or grid. Beyond that, inverters also monitor the energy produced by the panels and provide monitoring alerts if an issue is detected.
To interface with an electric grid, inverters must be connected to the grid and continuously monitor the incoming energy to match it with their output, thereby preventing power disruptions.
To convert energy from DC to AC, inverters use a combination of diodes and transistors built into their control boards. The inverter sends current through pairs of transistors rapidly, changing the direction of the energy. DC flows only in one direction, but AC rapidly switches directions, from positive to negative.
The US electric grid operates at 60 HZ (Hertz), meaning that the electricity makes a full “cycle” (zero – positive – negative – zero) sixty times per second.
Microinverter vs String Inverter
Inverters all perform the same task, so the difference between the two types is how they go about performing that task. In the sections below, we’ll look at a few of the key differences and how string inverters stack up against microinverters. It’s important to note that some brands of string inverters also employ module-level power electronics, also called optimizers, to enhance their operations.
Location & Number
By far and away the biggest difference between microinverters and string inverters is the location at which they are installed. Microinverters are attached to the underside of panels and are therefore installed on the roof. String inverters, on the other hand, are most often installed on the side of a home at ground level. Most commonly, they are installed next to the electric panel or meter for ease of connection.
Microinverter setups have one inverter tied to each panel, and string inverter systems have 1 inverter for the whole system. In some cases, large systems will have multiple two or three inverters.
Being that microinverter systems require more equipment than string inverter systems, using microinverters usually means the total system cost will be higher than that of string inverter systems. It also means that service is typically more expensive since replacing a faulty unit requires climbing onto a roof and removing one or multiple panels.
Reliability
Speaking of maintenance, reliability and maintenance costs are key areas of difference between the two inverter types. One of the most common arguments made against string inverters is that they offer a single point of failure. If the inverter goes down, the entire system shuts down. In microinverter setups, if one inverter fails, the other inverters can continue to operate.
In a previous study, we analyzed our claims data from 100,000 solar energy systems over five years. Our data showed string inverters having a failure rate of 0.89%, or 89 in every 10,000, and microinverter systems experiencing a failure rate of 0.0551%, or 5.5 in every 10,000 units.
That study, however, also noted that the cost to fix microinverter systems was higher and that the microinverters that did fail did so more quickly than the string inverters.
Performance
When it comes to performance, to get the whole picture, you need to look at both the system as a whole and the individual panel.
In string inverter systems, production is limited by the weakest panel. If one panel is shaded and produces less, it affects the output of the entire string. There are often 2-3 strings of panels per system. Voltage has to be the same in a given string, so if one panel underproduces, the rest need to reduce production to match it. Some string inverter brands utilize power optimizers to reduce or eliminate this effect, though.
Since microinverter systems have one inverter per panel, shade only affects the panels that are being covered. Every time energy is inverted from DC to AC, there are losses. Since the DC to AC conversion happens more in microinverter systems, the overall system efficiency is lower, meaning they generally produce less power.
Microinverter systems also lose power when batteries are involved because power must be converted from DC to AC at the panel, then back to DC to charge the battery, then back to AC to discharge from the battery. Some string inverters allow batteries to be DC-coupled, avoiding losses from an additional conversion.
Monitoring
Both string inverter setups and microinverter setups allow homeowners and businesses to monitor solar production, and many allow for consumption monitoring as well. Being panel-level, microinverters can display information from each panel individually.
String inverters that do not utilize power optimizers will only display total system production. If a system utilizes a power optimizer, then users can see panel-by-panel monitoring.
Scalability
The ability to add panels down the road is important to a lot of homeowners, as their needs will likely increase over time.
Scaling string inverters may or may not require new equipment, depending on what’s already installed. Most string inverters have a maximum input size, for example, 10 kW. If a homeowner only has 8 kW installed on a 10 kW max input inverter, they could add 2 kW without needing any additional inverter equipment. If a homeowner wanted to go above the maximum input value, they would need to add an additional inverter.
Scaling a microinverter system only requires a new microinverter that is compatible with the existing setup. This allows homeowners a great amount of flexibility in their setup, as long as the equipment is backward-compatible. If new generations of equipment aren’t backwards compatible, homeowners would need to change all the existing equipment to expand their system. The compatibility exception applies to power optimizers as well.
What’s The Better Choice?
At the end of the day, the better choice comes down to the goals of the individual homeowner. Both technologies have pros and cons in varying areas, so much so that we can’t call one definitively better than the other.
If your goal as a solar owner is to have a lower-cost system and your home is relatively unaffected by shade, string inverters may be a good choice for you. If your home has more trees around it and you want to scale in the future, microinverters may be the better choice.
While microinverters tend to be more reliable, owners may spend more down the road if they run into maintenance issues. Solar is a 25+ year investment after all, so issues are expected at some point.
Your chosen solar provider should work with you to explore your goals and design a system that truly meets your needs. If they’re a Solar Insure Certified Provider, they can also cover your chosen system with an SI-30 warranty to cover parts and labor for repairs down the line, making you even more confident in your investment.
Follow Solar Insure on our social media channels for all the information you could ever need about solar. Stay up to date with the latest in solar tips, trends, SI-30 product notes, and pro tips for success in your solar business.
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