Number of panels that can be fit in a single array with Fronius inverter

[guirgis]

First of all, I would like to clarify that I don't have solar experience except for some reasonable research over the past month. Also that I'm designing my grid-connected system chatting to a couple of installers so it's not a true DIY job.

After some thinking and researching, I've landed on the following high level system design:

• Placing around 11 panels on the north facing roof
• Placing around 6 to 7 panels on the east facing roof
• Preference is to go with Trina Vertex S+ panels, 430 or 435W so around 7.8 KW system
• Fronius inverter, Primo or GEN 24 model whatever's available.
• Looking to downsize the inverter to 6KW to maximise the efficiency as I've got a lot of trees around me so it's unlikely that the panels will be operating at 100%.

Here's a photo of the potential design - please note that this is an old satellite image as most of the trees on the north facing side of the house have been either removed or pruned.



I've asked for a quote from two installers, one of them sent me a quote and within a few hours they they came back identifying an issue with the pairing of Fronius and Trina, specifically on the number of panels (minimum and maximum) that can be in a single array referring to the MPPT voltage range and he suggested to swap the Trina for Jinko panels as they have a smaller OCV, I took this concern to the other installer but he doesn't seem to be seeing a concern at all and he keeps referring to previous installations that he's done and the only technical answer that I got that the Inverter operating voltage range is 600 V with no consideration to the MPP Voltage range.

I've included the response that I got from the installer where they detailed the issue, keen to hear your thoughts on the following:

• Does the MPPT voltage range affect the number of panels that can be fit in a single array or is it irrelevant? I've seen it linked more with battery connected PV systems.
• What are your thoughts on Fronius Primo vs Fronius Gen 24?
• What do you think of the Fronius/Jinko proposal?

I feel like I've gone in a rabbit hole and I'm looking for your help to get me out of it.

Many thanks,
Guirgis

Maximum voltage for the Fronius is 480 volts - we use MPPT voltage range from inverter spec sheet. Then we use (STC) VoC or Open Circuit Voltage from the spec sheet of solar panels.

Trina Panels are 51.8 voc per panel. 8 panels x 51.8 (x 1.1 for losses) = 455 voc per string. 9 panels is 512 voc per string and over the allowable voltage as per Fronius.
Jinko Panels are 39.38 voc. using the same calculations = min 6 panel, max 11.

See screenshots of data sheets below which are already linked into your proposal.

We could add a single panel to the eastern roof as a solution for the current Fronius 7.04kw design which I have adjusted now on your proposal.

Here's the proposal from the installer who identified the issue with the Fronius/Trina pairing.

 

[sunshine_eggo]

Rules you must follow:

1. Series Voc must be lower than 600V AND allow for temperature margin - 540Voc would be good down to about -2°C for typical panels.
2. Series Vmp must be lower than 480V.
3. Panels in a single string must be facing the same direction.
4. Strings in parallel must have highly similar Vmp.
5. String Imp must be less than 22A on MPPT1 and 12A on MPPT2, or it will be wasted.
6. String Isc must be less than 36A on MPPT1 and 19A on MPPT2, or the MPPT may be damaged
7. If you need to parallel two strings, follow rule 4 and put them on MPPT1.

 

[guirgis]

Thank you for your reply, appreciate if you can clarify the following:

1. Series Voc must be lower than 600V AND allow for temperature margin
2. Series Vmp must be lower than 480V.

Is this for a single string or for the whole system? My understanding that I'm after two strings, one with 11 panels and the other with 6.

3. Strings in parallel must have highly similar Vmp
May be a stupid question, does one of the photos suggest that I have strings in parallel?

Many thanks for your help!

 

[sunshine_eggo]

1,2 per string.

11S of 51.8Voc is 569.8V on one MPPT. If it ever gets below 12°C, you'll exceed 600V. On that basis the Jinko are the better choice for 11S. 10S Trina would be fine. If you have more space, and it kinda looks like you do, 6S (up to 10S) 2P on the first MPPT with the Trina would be a good choice.

6S on the other MPPT indicates the Trina is a better choice. The Jinko Vmp would be under peak power MPPT voltage of 230V. The Trinas would be > 250V.

3, no. Just a relevant rule for your hardware.

 

[guirgis]

Unfortunately not a lot of space on the east side, this is a duplex so I own only half of it, please see the photo below.



It sounds like with the limited space on the east facing roof where I can place 7 panels at best, Jink Vmp would still be under the 230 peak power so Trina would be a better option for that string.

Just trying to understand my chances of going Trina for both strings, what is the scenario where a string of 11 Trina panels would exceed the 600V? Is it something that I can derive from the datasheet?

Edit: I've attached the Trina datasheet.

 

[guirgis]

I found another thread that explains it.
So the temperature coefficient for Voc of Trina is - 0.24%/°C.
I've never seen it going below zero where I am, the coldest I've seen is 1 degree Celsius and I've been living in this house for ~4 years so a realistic temperature correction would be:

(25 - 1) * 0.24 = 5.76% so almost 6%

So with this correction, 11 Trina panels would go just beyond the 600 V:
11 * 51.8 * 1.06 = 603 V

Does this look right?

 

[guirgis]

And can I just understand, what is the scenario at which every cell in a string is operating at open circuit voltage?

 

[sunshine_eggo]

Unfortunately not a lot of space on the east side, this is a duplex so I own only half of it, please see the photo below.

As I was talking about 12+ panels, I was referring to the North face where it appears you could have more than 11 panels.

It sounds like with the limited space on the east facing roof where I can place 7 panels at best, Jink Vmp would still be under the 230 peak power so Trina would be a better option for that string.

I was not suggesting additional panels on the East face.

Just trying to understand my chances of going Trina for both strings, what is the scenario where a string of 11 Trina panels would exceed the 600V? Is it something that I can derive from the datasheet?

Edit: I've attached the Trina datasheet.

So with this correction, 11 Trina panels would go just beyond the 600 V:
11 * 51.8 * 1.06 = 603 V

Does this look right?

Yes, except for the temperature assumption.

I used a conservative assumption of 0.4%, and the Trina rating is better, so your calculation shows less voltage change than mine and is more accurate for the assumed temperature.

4 years is an insufficient timeframe. You need to investigate the weather extremes for your area. I've been at our location for 4 years as well. The lowest I've seen is 6°F (-14.4°C); however, on several occasions, NOAA historical weather data shows regular dips to -23°C with an all time low of -26°C over the last 30 years.

And can I just understand, what is the scenario at which every cell in a string is operating at open circuit voltage?

Light gives voltage.
Intensity of light gives amps.

Typically, the coldest time of day is just before dawn. The diffuse ambient lighting at dawn just before/after sunrise is typically enough to induce peak Voc before sufficient intensity exists to deliver current/power

With the Trinas, you need to go 10S or add a panel to the North face and go 6S2P.

 

[guirgis]

Thank you so much @sunshine_eggo, I think I didn't get your notation of using S and P, I now understand it. It's interesting that none of the installers I talked to every mentioned the option of mixing series and parallel circuits.

Interestingly, because I got conflicting datasheets for the inverter last week, I had contacted the Inverter local sales team in Australia and they just came back yesterday advising that the inverter voltage range is up to 1000V so there should be no issues now with going for 11S or more Trina panels even if I consider the lowest recorded temperature ever.


Only thing that I will have to have at least 6S Trina panels on the east face to operate within the MPP range of 240 -800. I'm a bit worried whether I have got enough space in the east face to fit the 6.

I'll be keen to hear your thoughts on whether I put the smaller string on the east or the west face, typically west gets more sunshine especially during summer but I've got a lot of gumtree on the west face hence why I initially thought of going east. Here's a photo from Google Earth that may give you an idea of the potential shading:



I really can't thank you enough for your help, it is very much appreciated!

 

[wattmatters]

keen to hear your thoughts on the following:

• Does the MPPT voltage range affect the number of panels that can be fit in a single array or is it irrelevant? I've seen it linked more with battery connected PV systems.
• What are your thoughts on Fronius Primo vs Fronius Gen 24?
• What do you think of the Fronius/Jinko proposal?

There are a few considerations for MPPT voltage. While others have commented on technical considerations, the 800-1000 V MPPT voltage limit (for the Fronius models here in Australia) is usually far higher than you need be concerned with.

The main restriction is a rooftop PV installation limit of 600 V DC applies in Australia. There are circumstances when arrays can go up to 1000 V DC but in general the 600 V limit still applies for most situations.

The previous generation Primo Snapinverter are a very solid inverter. Long track record in Australia and Fronius is the most popular inverter here by far. Fronius has a large presence here. The Gen 24 is a newer design but has been in the market for 2+ years. It is much quieter in operation using an improved cooling system and a larger fan. It can also be upgraded to be a hybrid (at this stage only compatible with BYD batteries) so that's a plus.

To be eligible for STC credits (solar rebate) the PV array's rated capacity must not exceed 133% of the inverter's AC output rating.

After some thinking and researching, I've landed on the following high level system design:

• Placing around 11 panels on the north facing roof
• Placing around 6 to 7 panels on the east facing roof
• Preference is to go with Trina Vertex S+ panels, 430 or 435W so around 7.8 KW system
• Fronius inverter, Primo or GEN 24 model whatever's available.
• Looking to downsize the inverter to 6KW to maximise the efficiency as I've got a lot of trees around me so it's unlikely that the panels will be operating at 100%.

Is there are reason you don't also want to put an array on the west roof, similar to the east roof array?

East + West would work well in parallel and this is when the Trina panels would be a better choice because their current is lower.

Honestly you get one good chance to get a grid tied system up, it's way harder later on. Put up the largest system you can fit now and you will thank me later.

West facing production is also more valuable than East - especially with peak time of use tariffs in Sydney kicking in from mid-afternoon.

Fronius have a nice system configurator tool:

Solar.creator

creator.fronius.com

According to Fronius configurator, you won't get an 11 panel string with either Trina or the Jinko panels. 10 panels is fine. Not sure why it won't accept 11 Jinko panels as the voltage is OK but that's what the configurator says.

I would be tempted to use an 8 kW Primo Gen 24 and have three strings of PV, 10 x North and (7 x East + 7 x West) in parallel.

This is a configuration using the Trina 430 W panels (but the 440 W panels would work as well) and an 8kW Gen 24 Primo:




There may be some Snapinverter Primos still available and the 8.2 kW unit is pretty decent, same configuration:



Going with Jinko may still be limited to 10 but you might get 11 on North.

And because of the lower voltage they operate at a higher current meaning parallel arrays are less favourable, hence the Trina would be much better as it will result in less current clipping in peak Summer.



But you may not be interested in having a larger system.

Even if you stick with the small system I would be going with a West array in preference to the East array. Unless there is something unusable with your West roof space.

 

[guirgis]

Thank you so much @Backup Down Under, good to hear from a fellow Aussie!

The main restriction is a rooftop PV installation limit of 600 V DC applies in Australia. There are circumstances when arrays can go up to 1000 V DC but in general the 600 V limit still applies for most situations.

I did hear that from one of the installers but I wasn't sure what it meant, especially the Inverter datasheet doesn't mention the 600V limit anywhere.

The previous generation Primo Snapinverter are a very solid inverter. Long track record in Australia and Fronius is the most popular inverter here by far. Fronius has a large presence here. The Gen 24 is a newer design but has been in the market for 2+ years. It is much quieter in operation using an improved cooling system and a larger fan. It can also be upgraded to be a hybrid (at this stage only compatible with BYD batteries) so that's a plus.

The ability to upgrade it later and connect it to a battery does make it future proof but I was just keen to hear if there are any noticeable differences between the two models when it comes to performance.

Is there are reason you don't also want to put an array on the west roof, similar to the east roof array?

East + West would work well in parallel and this is when the Trina panels would be a better choice because their current is lower

The only reason I'm in favour of east is due to the amount of gum trees that I have in front of the west facing roof - please see the photo in my previous post - winter production would be very little, however, summer would be better given the sun is higher and the long hours of afternoon sun.

I haven't thought of east + west, mainly due to being misinformed that I can only have either, none mentioned the option of parallel circuits and the suggestion was to choose one face, having been on the form for the last two days, I've got a few doubts on the installers knowledge.

East + West in parallel does seem like a better option but going for a larger system may not be worth it for me as I won't need that much much power and I don't have a battery to store that excess generated energy, the declining feed-in tariffs are not helping either. I'm also worried about the obstacles (skylight, antenna and ventilator) in those faces standing in the way.
I wonder if I can still go east + west but only with 4 each so overall the system would be:
10 S north, 2 x 4 P east + west? Would that work? I may play around with the tool and try to make sense of the results.

 

[wattmatters]

The only reason I'm in favour of east is due to the amount of gum trees that I have in front of the west facing roof - please see the photo in my previous post - winter production would be very little, however, summer would be better given the sun is higher and the long hours of afternoon sun.

Ah, I thought you mentioned trees having been cut back. You or the installer would need to do a shade analysis. You can have a play with Pylon Observer if you want:

Login | Pylon Observer

Fast and accurate solar designs with high resolution aerial imagery

observer.getpylon.com

Be a shame if West isn't viable as it's an ideal orientation, especially given Ausgrid's tariff regime.

The ability to upgrade it later and connect it to a battery does make it future proof but I was just keen to hear if there are any noticeable differences between the two models when it comes to performance.

There are subtle things. The main ones being the noise level and they will have different specs on the strings which can be fitted (probably different MPPT current limits), and the fact the Gen 24 can be a hybrid. Just be clear with an installer than you want a model which can be upgraded later - they have the option to unlock the feature with a paid upgrade and that's quite a nice option if you don't expect to need it for a few years.

I haven't thought of east + west, mainly due to being misinformed that I can only have either, none mentioned the option of parallel circuits and the suggestion was to choose one face, having been on the form for the last two days, I've got a few doubts on the installers knowledge.

You most certainly can parallel East with West. The most important factor is both array strings must be the same number and type of panels.

I would be seeking other quotes.

Don't rush this, take as much time as you need to learn and absorb info. Better to delay a little and get a better system design. If an installer won't take the time to discuss options/solutions with you or doesn't want to understand what you are trying to achieve, then I would move on to one who does.

The quality of installer trumps the quality of kit they are installing. Any kit qualifying for STC credits won't be completely awful but yes some kit is better than others.

East + West in parallel does seem like a better option but going for a larger system may not be worth it for me as I won't need that much much power and I don't have a battery to store that excess generated energy, the declining feed-in tariffs are not helping either. I'm also worried about the obstacles (skylight, antenna and ventilator) in those faces standing in the way.

All valid reasons, and things to discuss with a good installer. Consider relocation of some items (e.g. antenna) where possible. Solar real estate is valuable and it's worth optimising the space you have.

FITs will be modest in the years ahead and will likely be at a level which, if you 100% export, would mean the PV system breaks even over the long haul. So yes, self-consumption is what makes a big difference.

If the home has any gas appliances, they may eventually be replaced with electric alternatives (often much cheaper to run here). Water heating is an obvious item which can be mostly powered by solar PV.

Just keep in mind there is a strong chance you will have an EV in your driveway in the years ahead.

If you'd like to get a solid array of feedback/thoughts etc from Aussie grid tied PV owners, then the Whirlpool Energy forum is the place:

https://forums.whirlpool.net.au/forum/143

Aussies on here tend to be more of the off-grid / DIY variety. We are not permitted to install grid-tied stuff ourselves.

10 S north, 2 x 4 P east + west? Would that work?

Four panels is probably at the very low end of providing viable minimum string voltage and definitely the higher voltage Trina panel has the advantage in that case. It will work but I think you'd want to aim for longer strings if possible, higher voltages will help the MPPT.

There are other possible solutions, e.g.:

Primo 8.2 with two sets of parallel arrays:
6N + 6E
5N + 5W

The older Primo 8.2-1 has a 19A current limit for both MPPTs so that would work well too.

Anyway, much will depend on the shade profile of the roof.

 

[guirgis]

Ah, I thought you mentioned trees having been cut back. You or the installer would need to do a shade analysis. You can have a play with Pylon Observer if you want:

Login | Pylon Observer

Fast and accurate solar designs with high resolution aerial imagery

observer.getpylon.com

Not the gum tress as they are on a reserve, the ones that have been removed are on the north side.

Thanks for sharing the tool, looking at the satellite images helps with the decision indeed.

Be a shame if West isn't viable as it's an ideal orientation, especially given Ausgrid's tariff regime.

It's good in summer but poor when the sun is low, east seems to be better.

There are subtle things. The main ones being the noise level and they will have different specs on the strings which can be fitted (probably different MPPT current limits), and the fact the Gen 24 can be a hybrid. Just be clear with an installer than you want a model which can be upgraded later - they have the option to unlock the feature with a paid upgrade and that's quite a nice option if you don't expect to need it for a few years.

The future proofing of Gen24 is a big advantage.

You most certainly can parallel East with West. The most important factor is both array strings must be the same number and type of panels.

The installer today mentioned that it won't work having two arrays in different directions given the expected difference in generation between the two, it will operate efficiently only mid-day, it sounded reasonable, what are your thoughts?

I would be seeking other quotes.

Don't rush this, take as much time as you need to learn and absorb info. Better to delay a little and get a better system design. If an installer won't take the time to discuss options/solutions with you or doesn't want to understand what you are trying to achieve, then I would move on to one who does.

The quality of installer trumps the quality of kit they are installing. Any kit qualifying for STC credits won't be completely awful but yes some kit is better than others.

Thank you, I honestly feel like that I got into a rabbit hole though I do feel like I learned a lot over the past two weeks and I feel much more informed now.

All valid reasons, and things to discuss with a good installer. Consider relocation of some items (e.g. antenna) where possible. Solar real estate is valuable and it's worth optimising the space you have.

Would it make sense that the installer does this on the day of the installation? Seriously thinking of relocating the antenna, just need to get my neighbour's approval as it's shared by the other side of the duplex.

FITs will be modest in the years ahead and will likely be at a level which, if you 100% export, would mean the PV system breaks even over the long haul. So yes, self-consumption is what makes a big difference.

If the home has any gas appliances, they may eventually be replaced with electric alternatives (often much cheaper to run here). Water heating is an obvious item which can be mostly powered by solar PV.

Just keep in mind there is a strong chance you will have an EV in your driveway in the years ahead.

I got an EV two months ago and it's one of the biggest drivers for looking to get solar I don't have gas at home so that's another reason.

If you'd like to get a solid array of feedback/thoughts etc from Aussie grid tied PV owners, then the Whirlpool Energy forum is the place:

https://forums.whirlpool.net.au/forum/143

Aussies on here tend to be more of the off-grid / DIY variety. We are not permitted to install grid-tied stuff ourselves.

Thank you, this is very helpful!

Four panels is probably at the very low end of providing viable minimum string voltage and definitely the higher voltage Trina panel has the advantage in that case. It will work but I think you'd want to aim for longer strings if possible, higher voltages will help the MPPT.

You're right, 4 panels would be under the optimum MPPT range.

There are other possible solutions, e.g.:

Primo 8.2 with two sets of parallel arrays:
6N + 6E
5N + 5W

The older Primo 8.2-1 has a 19A current limit for both MPPTs so that would work well too.

Anyway, much will depend on the shade profile of the roof.

Thank you, I'll consider this along with the other smaller size option which is:
Two strings of 6 panels in parallel
One string of 6 panels, east or west facing subject to roof space/shading.

 

[wattmatters]

The installer today mentioned that it won't work having two arrays in different directions given the expected difference in generation between the two, it will operate efficiently only mid-day, it sounded reasonable, what are your thoughts?

What is the roof pitch? If it's a typical ~22° or less then they're talking out their backside (Fronius' own research shows this). The arrays are in parallel, one does well in the morning, the other does well in the afternoon and they barely affect each other. If the roof is steeply sloped then it's not as good but honestly the loss of production compared to having each facet on its own MPPT is bugger all.

MPP voltage doesn't change that much over a wide power band, while the current coming from each array will differ by quite a bit - that's the whole point of parallel arrays - the MPPT will sort it out, and the Fronius has a very good MPPT.

It's an old article but that doesn't make it any less relevant:

East West Facing Solar Panels On A Single Inverter Input Can Work Well

Wondering if you can put east and west facing solar panels on a single input inverter? The good news is you can - in certain conditions.

www.solarquotes.com.au

Heck, SMA even encourage it:

How to design East/West polystring PV arrays - Sunny. SMA Corporate Blog

Have you ever heard about polystrings? Did you know that PV plants with this special plant design benefit from more energy yield over the course of the day? Sunny Design Web will help you easily design a polystring PV Array. Find out all you need to know about this key but often overlooked feature.

www.sma-sunny.com

And on cloudy days with diffuse light it matters little which way panels are facing, they will all generate about the same.

The other option of course is Enphase microinverters. Then you can just fill the roof and have no need to be concerned with how many panels are on what facet, or an y need to have the same numbers of panels on each parallel array, or trying to staying within rooftop DC voltage limits. Enphase are considerably more expensive but it will eliminate design concerns and you can be less worried about partial shading issues.

For a battery later it can just be an AC coupled solution, something like Tesla Powerwall 2.

 

[guirgis]

What is the roof pitch? If it's a typical ~22° or less then they're talking out their backside (Fronius' own research shows this). The arrays are in parallel, one does well in the morning, the other does well in the afternoon and they barely affect each other. If the roof is steeply sloped then it's not as good but honestly the loss of production compared to having each facet on its own MPPT is bugger all.

Pylon is telling me it's 22.5 degrees.

MPP voltage doesn't change that much over a wide power band, while the current coming from each array will differ by quite a bit - that's the whole point of parallel arrays - the MPPT will sort it out, and the Fronius has a very good MPPT.

It's an old article but that doesn't make it any less relevant:

East West Facing Solar Panels On A Single Inverter Input Can Work Well

Wondering if you can put east and west facing solar panels on a single input inverter? The good news is you can - in certain conditions.

www.solarquotes.com.au

That's a pretty good read!

The other option of course is Enphase microinverters. Then you can just fill the roof and have no need to be concerned with how many panels are on what facet, or an y need to have the same numbers of panels on each parallel array, or trying to staying within rooftop DC voltage limits. Enphase are considerably more expensive but it will eliminate design concerns and you can be less worried about partial shading issues.

For a battery later it can just be an AC coupled solution, something like Tesla Powerwall 2.

I'm beginning to consider this although it's unreasonably expensive (almost 25% more), not for the shading benefit as I do believe Fronius does almost as good as Enphase but more for the flexibility that I'd get with Enphase, it's proven challenging to design a string a future-proof efficient system with the limited roof space (yesterday I discovered a ventilator that will prevent me for installing 6 panels on the east facet) whilst keeping within the optimum limits of voltage and amperage for the inverter and its MPPTs.