Why are there micro and string inverters instead of just wiring PV from the panels to the main inverter?

I've been searching a lot to try and understand the reason for micro and string inverters and I see a lot of comparisons between the two types but I can't find anything that explains their purpose. Some things I have discovered as a benefit but not necessarily a reason for their existence:

1. Micro inverters can help with the performance of an entire array of panels.
2. Micro inverters can provide statistics on each panel and report things like how much power of panel generates.
3. String inverters are generally less expensive but I've seen arguments otherwise when you factor in things like longevity and warranty cost.

Is the main reason for safety because if you have to perform maintenance you will be handling cables with AC instead of DC? What is the core reason for these inverters instead of just wiring your PV cables down to a combiner box which then goes down to a main inverter(s)?

Do some math on how much that wire will cost in labor and materials compared to an AC or DC string, for EG a 10-20 panel home system and you’ll see…

For larger systems it just escalates from there.

EDIT: Also there is often an optimal tradeoff between current and voltage for various electronics components. 10 solar panels in parallel is over 100A which is well outside that sweet spot

String inverters IS wiring of PV panels directly to the main inverter.
Sometimes you parallel 2 or string inverters for more watts.

Microinverters are just tiny low DC voltage versions of string inverters.
Biggest advantage is that with a "string" of one PV panel, don't need to worry how many are oriented the same.
Easy to populate any roof with all that fit. That's the main reason.

String inverters are less expensive, more efficient, and not put in such a harsh environment.
But RSD has brought cost closer to what microinverters cost. If you pay a bit more for optimizer, then you get per-panel statistics.

One other kind is SolarEdge - they distribute part of the string inverter function to per-panel electronics.

Primarily, it is competition between engineering (and marketing) approaches. With a bit of special-interest lobbying thrown in.

Not a fan of microinverters in Australia- with summer temps in the north hitting 40C plus for weeks on end (100F plus)- putting electronics just beneath panels that can hit 80C plus- well- yeah- not a good idea (I have worked on arrays that have had units fail in under a year, some with the entire arrays microinverters having to be replaced before five years was up...)
Even string inverters- I prefer if possible to have them on the southern wall of the building to keep the summer sun off them, or preferably inside in the garage if it is an attached one... or otherwise have a 'shade cover' fitted over them

zanydroid said:

Do some math on how much that wire will cost in labor and materials compared to an AC or DC string, for EG a 10-20 panel home system and you’ll see…

For larger systems it just escalates from there.

EDIT: Also there is often an optimal tradeoff between current and voltage for various electronics components. 10 solar panels in parallel is over 100A which is well outside that sweet spot

Click to expand...

I don't understand this answer. I'm trying to understand the reason that these micro inverters and string inverters were conceived. Wiring cable directly from the panel to the main inverter would cost a lot less than adding these individual devices to the overall cost.

Hedges said:

String inverters IS wiring of PV panels directly to the main inverter.
Sometimes you parallel 2 or string inverters for more watts.

Microinverters are just tiny low DC voltage versions of string inverters.
Biggest advantage is that with a "string" of one PV panel, don't need to worry how many are oriented the same.
Easy to populate any roof with all that fit. That's the main reason.

String inverters are less expensive, more efficient, and not put in such a harsh environment.
But RSD has brought cost closer to what microinverters cost. If you pay a bit more for optimizer, then you get per-panel statistics.

One other kind is SolarEdge - they distribute part of the string inverter function to per-panel electronics.

Primarily, it is competition between engineering (and marketing) approaches. With a bit of special-interest lobbying thrown in.

Click to expand...

I have 22x100W panels on my roof (5Sx2P, 6Sx2P) and all wired directly down to the combiner box and then to the main inverter. I did all this before I came across the concept of micro inverters and string inverters hence I was asking. Everything is working just fine for me so I was just curious why this additional equipment was ever introduced in the first place.

adirondack_wanderer said:

I don't understand this answer. I'm trying to understand the reason that these micro inverters and string inverters were conceived. Wiring cable directly from the panel to the main inverter would cost a lot less than adding these individual devices to the overall cost.

Click to expand...

Hello, They are an option because not everyone's install is the same. A string inverter often is the main inverter, so I'm not following that part of your question.

• String inverter is sometimes cheaper, great for un-shaded ground mount arrays. Less parts.
• Micro inverters are great for roof mounts, especially with some shade since shade on one panel doesn't decrease the output of the entire array as much. They also allow panels to be shut down on an individual level in case you ever have a fire, and responders have to be near the array.

It's generally a good thing to have choices. And those choices generally exist because we don't all have the exact same setup.

Does that help?

adirondack_wanderer said:

I don't understand this answer. I'm trying to understand the reason that these micro inverters and string inverters were conceived. Wiring cable directly from the panel to the main inverter would cost a lot less than adding these individual devices to the overall cost.

Click to expand...

String inverters will always be cheaper to wire as a string than as all panels paralleled together.

I don’t know about the answer before rapidshutdown, but theoretically if you wanted to parallel all panels you would need to thread the code needle carefully. For instance individual solar panels would have to be under 30V Voc.

The wire size for 100A parallel circuits @30V would be difficult to pass through home framing and require more careful planning than a much smaller number of 240-500V circuits. It would also need to be significantly increased in size to compensate for the very large % voltage drop.

adirondack_wanderer said:

I have 22x100W panels on my roof (5Sx2P, 6Sx2P) and all wired directly down to the combiner box and then to the main inverter. I did all this before I came across the concept of micro inverters and string inverters hence I was asking. Everything is working just fine for me so I was just curious why this additional equipment was ever introduced in the first place.

Click to expand...

Shade tolerance as other post said.

Small roof planes - with modern RSD you can often need 6-7 panels in a series set to match the installation instructions. High voltage MPPT generally do not scale down that far either. And with UL9540 multiple MPPTs all need to be listed for UL9540, and there are very few UL9540 systems that include separate MPPTs

Simplified design work for solar companies. String inverters are a lot easier to misdesign than microinverters.

zanydroid said:

String inverters will always be cheaper to wire as a string than as all panels paralleled together.

Click to expand...

The reason I said paralleled together because this is necessary for a string inverter to somewhat (but not exactly) match the shade tolerance of microinverters.

adirondack_wanderer said:

I don't understand this answer. I'm trying to understand the reason that these micro inverters and string inverters were conceived. Wiring cable directly from the panel to the main inverter would cost a lot less than adding these individual devices to the overall cost.

Click to expand...

String inverters ARE a 'main inverter' ie usually one inverter (usually at ground level) with either one or two series strings (many have dual inputs for the larger inverters)- while a micro inverter is the ones up on the roof behind the panels- these do have their advantages- especially in areas where you get partial shadowing of the array during the day- but they also have disadvantages (heat is a killer on electronics, which is why you don't find many people using them in Australia- especially inland or up here in the north- they fail very quickly)

Bop said:

heat is a killer on electronics, which is why you don't find many people using them in Australia- especially inland or up here in the north- they fail very quickly

Click to expand...

While that's definitely an issue, there are plenty of Enphase systems in SEQ and elsewhere. It's gets hot but not like it does inland.

Designing electrical stuff in places like where you are does need care because of the prolonged and at times extreme heat. I think newer models have got better at working in the heat but no matter what it is, the hotter it is, the shorter its life. Changing out a failed micro is a PITA.

I think the main reason we don't see as many microinverter systems in Australia is cost. Expect to pay 25-75% more for the same system capacity if you choose microinverters.

We don't have the same rapid PV shutdown requirements as the USA (which when invoked brings the array voltage right down low). If we did then our string arrays would cost significantly more, and be closer in cost to microinverters.

Microinverters are good for systems which cannot easily form strings of suitable lengths for a string inverter's MPPTs. Handy for McMansions with their stupidly complex roof facets.

Shade tolerance gain is marginal, modern string inverters and panels are quite good at dealing with transient shading.

Either can provide a very good system, provided it is designed and installed well. This matters more than the type of inverter.

SunDave said:

Hello, They are an option because not everyone's install is the same. A string inverter often is the main inverter, so I'm not following that part of your question.

• String inverter is sometimes cheaper, great for un-shaded ground mount arrays. Less parts.
• Micro inverters are great for roof mounts, especially with some shade since shade on one panel doesn't decrease the output of the entire array as much. They also allow panels to be shut down on an individual level in case you ever have a fire, and responders have to be near the array.

It's generally a good thing to have choices. And those choices generally exist because we don't all have the exact same setup.

Does that help?

Click to expand...

Thank you for being patient with me. I wasn't aware that a main inverter, like an EG 6000xp, was also called a string inverter, or perhaps it's always been called string inverter but with my limited knowledge I referred to it as a main inverter.

I'm still not clear how a micro inverter helps an array of panels that may experience shade. If a panel is shaded then it's not going to perform well whether it is tied to a micro inverter or not, correct? Is that the reason micro inverter's were created because the problem to solve was shaded panels impacting a larger array? I do understand the other point you made where it allows the panel to be shut down in the event of a fire. Would that be ideally a wireless control to shut it down?

Microinverter, if its panel is shaded, doesn't make AC.
Has no impact on the other microinverters.
So convenient for designer/installer to put 1 microinverter on one roof face, 19 microinverters on a different face.

String inverter, all panels in series affect each other.

RSD, just turn off AC going to all the microinverters. No benefit to wireless. String inverter, have to send powerline signal or some other means for keep-alive. Microinverter could use powerline or wireless for monitoring and updates.

Many people believe that a single shaded panel in a series string will 'kill the string' and stop it making power...
However that really isn't the case- you lose the output of that panel, but the rest of the string will continue to produce power...

A while back I actually took some photos of my own system doing it (after I had one guy was determined that a shadows series string would have no output at all lol) the 3 panels on the right are a series string...




Some claim they last longer than string inverters- well that certainly hasn't been our experience with them in Australia- its extremely rare to see a microinverter array with all (or even some) of all its inverters still functioning in 5 years, where string inverters are commonly seen lasting 10-15 or even 20 years plus and still powering on...

(who would have thought that having electronics that hate heat and it causes them to fail prematurely- mounted in a box in 40C plus air temps, right behind a panel running at 80C plus, would fail prematurely???)

I know, its a mystery isn't it lol

adirondack_wanderer said:

I'm still not clear how a micro inverter helps an array of panels that may experience shade. If a panel is shaded then it's not going to perform well whether it is tied to a micro inverter or not, correct? Is that the reason micro inverter's were created because the problem to solve was shaded panels impacting a larger array? I do understand the other point you made where it allows the panel to be shut down in the event of a fire. Would that be ideally a wireless control to shut it down?

Click to expand...

You are correct that microinverters are arguably squeezing water from a rock. That said, suppose a panel still has 50-66% production after shading. Do you want to harvest that? Maybe. The MPPT algorithm is also likely simpler on a microinverter. There are some shading orientations where you can get more from the shaded cells with micros than with string inverter.

Overall there are fewer design issues that can happen with micros versus strings, but you could also argue that this is just helping lazy installers.

The rapid shutdown is either wireless or PLC (signal modulated onto the power conductors by electromagnetic wiggle) based.

adirondack_wanderer said:

Thank you for being patient with me. I wasn't aware that a main inverter, like an EG 6000xp, was also called a string inverter, or perhaps it's always been called string inverter but with my limited knowledge I referred to it as a main inverter.

I'm still not clear how a micro inverter helps an array of panels that may experience shade. If a panel is shaded then it's not going to perform well whether it is tied to a micro inverter or not, correct? Is that the reason micro inverter's were created because the problem to solve was shaded panels impacting a larger array? I do understand the other point you made where it allows the panel to be shut down in the event of a fire. Would that be ideally a wireless control to shut it down?

Click to expand...

Hi Adirondack,

In regards to shade:
This is how I think of the micro-inverter. It may not be technically accurate, but it helps to visualize.

The micro-inverter makes each panel and micro-inverter a small separate system. So lets say 10 panels and 10 micro-inverters are 10 very small systems then wired together.

The string inverter would be the same 10 panels, but acts more like 1 simple system.

In this case, the simpler system is cheaper, but if your chimney blocked a large part of a panel with shade, the micro-inverters are impacted less, and the string inverter is impacted much more. The chimney only impacts 1 or two small systems on the micro-inverters, but impacts the string inverter system a whole bunch since the lower production can drag the other panels down with it.

So you are correct. A shaded panel will not perform well on either inverter, but one shaded panel on a string inverter can lower a whole string more.

---------------------------------------
In regards to safety:
Micro-inverters sometimes communicate through the wire that already goes to the panels (even power wires can carry some signals) or wirelessly like you suspected.

Many string inverters also have safety features. Arc protection, lightning protection, a button to shut down the whole inverter/battery system, and that type of thing.

The interesting thing is, that you don't have to choose. Some micro-inverters where shade could be an issue, and string inverter where it is not an issue. Some solar devices even accept both inputs into the same device.

SunDave said:

The chimney only impacts 1 or two small systems on the micro-inverters, but impacts the string inverter system a whole bunch since the lower production can drag the other panels down with it.

Click to expand...

Unless you are using old technology, it doesn't work like this any more.

Modern PV modules are designed to automatically bypass shade affected sections of modules so as to significantly reduce impacts on the rest of the string, and modern well designed MPPT systems work well to enable this management to significantly reduce the overall impact on array production.

wattmatters said:

Unless you are using old technology, it doesn't work like this any more.

Modern PV modules are designed to automatically bypass shade affected sections of modules so as to significantly reduce impacts on the rest of the string, and modern well designed MPPT systems work well to enable this management to significantly reduce the overall impact on array production.

Click to expand...

REALLY old tech panels (bypass diodes have been a feature of practically all halfway decent panels since the late 1980's,early 1990's)

Bop said:

REALLY old tech panels (bypass diodes have been a feature of practically all halfway decent panels since the late 1980's,early 1990's)

Click to expand...

I was thinking of that + half cut cells which only started being produced ~10 years ago.