String inverter vs microinverter: 4 reasons microinverters are better for your system.

Microinverter vs String inverter

Microinverters are a newer breed of inverters that were created to tackle some of the issues presented when using traditional string inverters.

But are microinverters the better option for your system?

In this article we help you answer this question by looking at:

  • How both string inverters and microinverters work.
  • The differences between these inverter technologies in cost, lifespan, and maintenance.
  • And when you should consider using microinverters for your solar energy system.

Key differences between string inverters and microinverters.

To understand the difference between these two technologies and which one will better suit your needs, we’ll take a look at how each of them works while pointing out the limitations of a string inverter.

But before we get into that, here are a couple of things you should know:

What is a solar panel string?

A solar panel string is a set of solar panels wired together in series. The voltage across the “string” equals the sum of the voltages across each panel, while the current in the string is the lowest current produced by any panel.

For example, if we have 8 solar panels in series and one panel is shaded and producing low current, the current in the string will equal the current from that particular panel.

What is Maximum power point tracking (MPPT)?

Maximum power point trackers – or MPPTs – are used to maximize the power output of your solar panels. This is done by matching the output voltage of a solar generator with the input voltage of the grid (in grid-tie systems) or the battery (in Off-grid systems).

What does a string inverter do?

A string inverter connects to one or multiple strings, optimizes the power produced by these strings, and then turns the DC (Direct Current) into a more usable AC (Alternating current).

Each of these strings consists of solar panels facing the same direction, titled at the same angle, and of the same type.

The number of strings that can be connected to a particular string inverter depends on the number of string inputs (DC connections) it has.

The number of solar panels per string depends on the starting voltage and max input voltages of the inverter.

For example, the datasheet shown below is from a Fronius string inverter the inverter has a max DC voltage input of 1000, which means each string cannot exceed 1000V of DC voltage.

The datasheet also specifies “2+2” as the number of DC connections, meaning that the string inverter has 2 MPP trackers with 2 DC inputs for each tracker (4 DC inputs in total).

Each MPP tracker will search for the global maximum power point. If one string is connected to the MPPT, the latter will look for the voltage for which the string produces maximum power.

If two strings are connected to the MPPT, it will look for the global voltage that results in the maximum power point of those 2 strings, and so on.

This doesn’t sound so problematic, but Since the MPPT searches for the global voltage that provides the maximum power, differences between the strings such as:

  • Different lengths: different number of solar panels.
  • Different levels of solar irradiance: solar panels facing different directions, tilted at different angles or experiencing different levels of shading.
  • Different types of solar panels: different levels of degradation, different energy production, etc.

Would result in inefficient MPP tracking.

Unless all the strings connected to one MPPT are identical and produce the exact same power at all times (which in the real world is almost impossible), there will be a few percentage points of energy production losses.

If the open-circuit voltage of each string does not exceed the rated max DC input voltage of the inverter, and strings connected to each MPP tracker are identical, the string inverter would work like a charm.

Of course, As long as all the solar panels in each string produce the same power.

However, due to uneven shading, uneven panel degradation, uneven soiling, etc., the solar panels on the same string will not always have the same output.

For example, if one module is blocked from sunlight (because of shade, leaves, bird poop, etc..) it starts acting as a resistive load, which results in a lower current flowing through the string.

So even if all the solar modules on the string match, there will always be a possibility of one solar panel dragging down the production of the whole string.

Now let’s see how microinverters solve these issues.

What does a solar microinverter do?

Microinverters do both the optimizing and the inverting on the solar panel level, and the converted power goes directly to the AC bus.

This means that the panels are independent of each other, and do not have to be of the same type, face the same direction, or have the same tilt angle.

Each solar panel would have a microinverter installed under it; each microinverter has an integrated MPP tracker which allows the tracking of the maximum power point of each panel individually.

The optimized output is then converted from DC to AC and connected to the AC bus.

String inverter vs microinverter: Key differences.

Since Enphase is the most established and reliable microinverter manufacturer, we’ll compare their microinverters with some of the most established inverter manufacturers in the market.

Cost

Typically, it’s cheaper to buy and install string inverters. For more context, we’ll use the average size of a residential solar installation in the US (5kW system).

The table shows how much a 5kW string inverter and microinverters will cost.

Manufacturer Type Model Price
Fronius String inverter Fronius Primo 5.0-1 TL inverter $1600
SMA String inverter SMA Sunny Boy 5.0-US inverter $1400
Enphase Microinverter IQ7+ (17 total microinverters for 17-300W panels) $2600

Lifespan and warranty

Manufacturer Type Model Warranty period
Fronius String inverter Fronius Primo 5.0-1 inverter 10 years
SMA String inverter SMA Sunny Boy 5.0-US inverter 5 years
Enphase Microinverter IQ7+ 25 years

Production monitoring

Like most inverters, Enphase microinverters come with a system production monitoring platform for homeowners (MyEnlighten). These monitoring platforms allow you to monitor the overall system production and performance.

However, unlike traditional string inverters, Enphase also offers solar panel-level monitoring for solar professionals (your solar installer) through the Enlighten Manager app, which makes for easier maintenance and management.

And if you’re a DYI person, Enphase also offers solar panel-level for homeowners through an upgrade. However, the upgrade will cost a one-time fee of $249 or a monthly subscription for $9.99.

Maintenance

In power electronics, the more complex the electronic circuitry is, the higher the chance of failure. In a PV system, inverters have a higher chance of failure compared to solar panels.

If a string inverter fails, the fact that it’s mounted on the ground level makes it easier to diagnose and potentially fix.

But if your system is underperforming due to loose or damaged wiring, solar panel degradation, Enphase’s monitoring platform can come in handy.

Having access to panel-level production can make the identification of these failure modes a much easier task.

All things considered, it’s safe to say that microinverter do a way better job than string inverters. But in most cases, string inverters will be the most cost-effective option.

But there are some cases where microinverters are the better option.

String inverters vs microinverters: 4 reasons microinverters are worth it.

Microinveters are relatively a newer technology that was introduced to solve some of the issues the traditional string inverter presented. But do you really need them?

despite all the perks of Enphase’s microinverters, in most cases, string inverters are currently still the most cost-effective solution for your solar system.

But there are some cases where the usefulness of microinverters is undeniable.

Efficiency in partial shading:

As depicted above, the Achilles heel of a string inverter is the way the solar panels are wired together. The power production of all the solar panels can be dragged down by the underperformance of one.

If you have shading on your roof, a string inverter is just not going to cut it for you. Because if shade blocks even just a couple of cells from one panel, the whole string’s performance is going to be affected.

With microinverters, the MPP tracking is done on the solar panel level, so if one panel is shaded the others panels are still going to produce maximum power.

This doesn’t mean that microinverters will totally solve your shading issues. But if you do have shade, microinverters can mitigate the effects of shading on the whole system.

In fact, this NREL study shows that when a PV system is under different levels of shading, around half of the performance can potentially be recovered by the use of microinverters.

For example, if a solar installation equipped with a string inverter is underperforming by 20% because of partial shading, the same installation would only lose about 10% if it were equipped with microinverters instead.

This study was conducted by applying different degrees of partial shading to 2 identical solar installations. One of the installations was equipped with a string inverter, while the other with a microinverter on each solar panel.

In your case or any, the potential production recovery depends on the shading intensity and the location of the shading on the system.

In this case, the results of this experiment showed a production recovery of up to 12% under heavy shading.

Better for complicated roof structures:

This is in my opinion the best-case scenario for using microinverters and where their benefit is very clear.

For example:

Let’s say you want to install 36 panels on your roof, and you have 3 viable roof sections or facets where you can install these panels.

The south-facing section has enough usable space for 20 panels, the west-facing section can fit 6 panels, and the east-facing section can fit up to 15 panels.

Between these 3 sections, you have more than enough space to fit all the solar panels you need; the problem is, these sections are facing different directions and possibly different pitch angles.

This means the strings of panels on each side will produce different amounts of power throughout the day.

  • In the morning (5:00-10:00): the panels on the east-facing section will produce the most amount of power.
  • Around noon (10:00-15:00): the panels on the south-facing side will produce the most amount of power.
  • In the afternoon (15:00-19:00): the panels facing the west will produce more power than the other panels.

If you use string inverters in this case, you’ll need 2 or 3 string inverters.

Following our example, if we install 20 south-facing solar panels, 10 panels facing east and 6 facing south, it should be 3 seperate strings of solar panels.

Unless you have a string inverter with 3 MPPTs or 2-3 string inverters with an MPP tracker for each string, there would be a considerable amount of performance losses.

With micro-inverters deployed on each solar panel, the solar panels in your system can all be facing different directions and have different tilt angles. Each of them would still produce maximum power.

Related topics:

Easier system expansion:

Another good case where the utility of microinverters is rather obvious is the flexibility that they create.

For any reason, If you want to start off with a few panels and scale slowly, microinverters would allow you to expand your solar system without having to plan ahead.

If your system is equipped with microinverters, you wouldn’t have to worry about mismatched solar panels, angle, or azimuth, if you want to expand. This makes a big difference.

Warranty and longevity:

While string inverter manufacturers like SMA and Fronius offer warranties of 5 to 10 years, Enphase – the leading microinverter manufacturer – offers a standard 25-year warranty for its microinverters,

This tells you a little bit about how long Enphase expects their microinverters to stay operational.

The upfront cost of a string inverter might be cheaper, but if it becomes defective outside of the warranty period, you’ll have to buy a new one.

Key takeaways:

  • Microinverters are better for your system if you have shading issues or if you have a complicated roof structure.
  • Microinverters are more flexible when it comes to system expansion.
  • If you have none of the issues above, string inverters are the cost-effective option for your system.
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Younes
Younes

Hi! My name is Younes. I'm an electrical engineer and a renewable energy enthusiast. I created renewablewise.com with a mission of delivering digestible content and information to the people who seek it.

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