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String Inverters, Power Optimizers and Microinverters

An inverter is the heart of a solar installation.

While solar panels would convert sunlight into DC electricity, an inverter would change the DC electricity into AC electricity usable by home appliances.

String inverters, power optimizers and microinverters are major types of the latest solar power electronics, each with different strengths and weaknesses.

We outline the benefits and drawbacks of each technology to help you make the right inverter decision for your solar investment.

String inverters, power optimizers and microinverters differ in the way power electronics are located, and here is a summary of their pros and cons.

Inverter typesProsCons
String invertersEasy to maintain
Proven technology with predictable reliability records
Low cost
Only string-level monitoring
Only string-level MPPT
Difficult to expand system size in the future without buying a new inverter
Power optimizersIndividual panel MPPT is a great solution to partial shading
Panel-level monitoring available
Panel-level rapid shutdown
Difficult to expand system size in the future without buying a new inverter
Difficult to maintain with partial electronics on the roof
MicroinvertersIndividual panel MPPT is a great solution to partial shading
Panel-level monitoring available
Panel-level rapid shutdown
Easy to expand system size in the future
Safer, low operating AC voltage compatible with the grid
Most expensive
Difficult to maintain due to electronics on the roof

We will go through each technology in detail.

What Are String Inverters?

String inverters are the most popular inverter type on the market, with the most mature power electronics technology.

Major solar inverter brands such as SMA, Delta, Fronius and KACO made their names from manufacturing central and string inverters that convert DC energy from solar panels into AC energy that is compatible with the grid and powers home appliances.

string inverter
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How Do String Inverters Work?

In a string inverter solar installation, only one inverter is needed per string of solar panels.

The whole string of solar panels is wired in series to reach a few hundred volts to drive the inverter.

And the inverter itself can optimise the performance of the whole module string through its MPPT algorithm, to make sure the solar panels operate at the maximum power no matter the weather conditions.

The string inverters are usually installed indoors at the ground level in an accessible location, therefore string inverters are known to be quite easy to maintain.

How Can Partial Shading Impact Solar Installations?

Since string inverters only optimise solar panel production at string level, whenever partial shading occurs, such as bird dropping, trees, or a chimney, the MPPT algorithm has trouble separating the shaded module performance from unshaded modules.

As a result of the module mismatch, the output of the module string is dragged down by the lowest performer, reducing the production of otherwise perfect modules.

What Is MLPE?

MLPE stands for Module Level Power Electronics, and it is a generation of solar inverter technologies that can solve the shortcoming of string inverters, such as partial shading.

MLPEs solve the partial shading issue by installing power electronics at every solar panel, and thus providing a finer MPPT control at each solar module.

What Are the Key Benefits of MLPE?

Since MLPEs have dedicated electronics at each solar module, the electronics can perform MPPT on each panel individually, with other features including measuring power outputs and rapid shutdown.

Module-level MPPT

Module-level MPPT can reduce the impact of module mismatch caused by individual panel shading, soiling, by isolating the low power module from the perfect ones.

This way, when any particular module is shaded, other unobstructed panels can still operate at full strength, instead of being dragged down like the string inverter system.

Module-level monitoring

MLPEs can measure the performance of individual panels, which leads to more sophisticated monitoring systems that let owners see the output of each panel, and makes it easier to pinpoint module defects when troubleshooting.

Rapid shutdown

A common problem with solar systems is the risk of electrocution to firefighters in the event of a fire during the day.

By having dedicated electronics at each panel, the MLPEs can “turn off” the solar panels by way of rapid shutdown.

This provides a safer provision for firefighters in the event of a fire.

Major types of MLPEs include power optimizers and microinverters, and we’ll describe the differences between the two below.

What Are Power Optimizers?

Power optimizers are MLPEs that separate the MPPT and monitoring circuitry from the inverters.

An MPPT and monitoring module is installed at every solar panel that optimises the panel individually to minimize shading and mismatch impact on the system.

Major power optimizer brands include SolarEdge and Tigo.

power optimizer
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How Do Power Optimizers Work?

Power optimizers’ MLPE circuitry converts the unregulated DC output of solar panels into steady DC energy fed into a string inverter.

Apart from the additional optimizer between each panel and the inverter, the architecture is essentially the same as a string inverter system.

Therefore, a SolarEdge inverter can be installed at ground-level for easy maintenance, while the less complex MLPE circuits stay on the roof, and have similar reliability as the solar panels.

Though the event is rare, in the event of an optimizer failure, it is a costly exercise to replace a power optimizer on the roof.

What Are Microinverters?

Microinverters are full-fledge inverters that operate at a lower voltage and low power output that is installed on the back of each solar panel.

Each microinverter only handles the power of a single panel, so they are built in a smaller and more optimized package, resulting in better reliability than string inverters.

The dominant microinverter brand on the market is Enphase.

microinverter
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How Do Microinverters Work?

The microinverters are designed in a small package that can easily install on the back of each panel, creating the benefits of MLPE.

Each microinverter converts a solar panel’s DC output directly into grid-compatible AC, and parallel each panel in the system to boost the overall size.

This parallel connection is different from the traditional string inverter system design and has the benefit of avoiding string-sizing headaches and makes the system much more scalable.

Despite microinverters having the benefits of MLPEs and scalability, the complexity of electronics on the roof makes them more expensive and less reliable than the alternatives.

Which Inverter Type Should You Choose?

The choice of which type of inverter to use depends on how well the application matches the inverter benefits.

Inverter typesSuitable applications
String invertersUtility, commercial and residential
Power optimizersCommercial and residential
MicroinvertersCommercial and residential

The cost-sensitivity and the ease of maintenance make string inverters a great all-rounder, and especially suitable for large-scale, utility and commercial installations.

Power optimizers are suitable for rooftop applications that want to maximise power output despite the shade, and also benefit from detailed monitoring features.

Microinverters are best suited for rooftop applications prone to shade, that are difficult to string-size panels and need to install panels at different angles and orientations near trees.

To learn more about how WINAICO solar technologies can help with your rooftop energy production, please get in touch with us.

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