Just got a 9.6kW system installed and does not peform to expected power output.

[space4092]

Hi there,

I just got a 9.6 kW system installed on our house by a contractor, they designed the system.

The Solar panels are Q.PEAK DUO XL-G10.3 480

Specs:

Nominal power (W) - 480
Short circuit current (A) - 11.26
Open circuit voltage (V) - 53.61
Current at maximum power (A) - 10.71
Voltage at maximum power (V) - 44.81
Maximum system voltage (V) - 1500

The Inverter that was installed is a Sunny Boy 7.7-US.

The solar panels are connected in 3 strings. 1st with 8 panels, 2nd with 7 panels and 3rd with 5 panels.

15 panels are facing south and 5 panels facing west.

As per plate they installed on the inverter DC output rating is 9.6 kW - AC output rating is 7.7 kW.

The peak power production since the system was put online was 5kW.

So I do not understand why the strings are so uneven. wouldn't you want even strings for no power losses? I got a system up north that I put together and I have 10x100 watt panels in series-parallel configuration with 2x 5 panels in series and those wired in parallel. Therefore there are no power losses. The peak on my system is ~960 watts, with all panels facing south.

I get that I shouldn't see 9.6kW on peak but shouldn't the peak be at least 7.2kW when all 15 south-facing panels are at peak production? or is the voltage drop due to the west panels? I assume that they are on the string with 5 panels. and the two other strings are the south-facing ones.

or should the west panels really be on their own power inverter? and have equal strings on this inverter of 5 panels each with all facing south and then getting the 7.2kW peak?

Why is the AC output rating limited to 7.7 kW? is the inverter undersized for a 9.6 kW system?

It just does not make sense to me that they would design such a poor system. I am missing something? Should the MPPT controller make up for this?

Please help me understand what is going on here. The system that I build makes sense and performs as expected, their system does not make sense and does not perform as expected.

Thanks!

[J.P.M.]

I'd start with a call to the installer and ask the same questions, especially since they designed the system.
Provided the installer is reputable and professional, that's probably also the best place to get information about what's going on with your system.
Since you have experience with system design, did you review the information supplied for code review and approval? Seems like that would have included string wiring info.

Besides, servicing the system under warranty is part of what you paid for.
The string sizing does seem a bit funky, but given the info you've provided, without being there it's not possible to get an idea of what's going on (or not going on).

While not disputing your claim about your 1kW system's performance, to your question, or at least a partial answer to your question about whether or not 7.2 STC kW of south facing panels by themselves ought to be producing (a maximum ?) of 7.2 kW of power, the answer is that it's pretty unlikely.
Reason: Panels in situ will rarely achieve their STC rated output.

Also, the max. system voltage you state = 1500 V?
Per the inverter spec sheet max. voltage = 600 V.

[bcroe]

Also, the max. system voltage you state = 1500 V?
Per the inverter spec sheet max. voltage = 600 V.

1500V is the panel element to frame insulation operating rating. Bruce Roe

[oregon_phil]

These are bifacial solar panels. How are they mounted? If roof mounted, what rapid shutdown devices are used?

I have an SMA 7.7-40 and have no issues with different string lengths on each MPPT. As long as electrical specification are met, you shouldn't have issues with this string inverter.

[scrambler]

Do you have individual panel monitoring?

[SolTex]

Why is the AC output rating limited to 7.7 kW? is the inverter undersized for a 9.6 kW system?

It is common practice to undersize the inverter. Many system designers will shoot for a 1.25 DC/AC ratio (Inverter Loading Ratio).
Your ratio is 9.6/7.7 = 1.25 so right on the mark.
If you are curious, do a Google search for Inverter Loading Ratio (ILR) to learn more.

[davidcheok]

Hi there,


So I do not understand why the strings are so uneven. wouldn't you want even strings for no power losses? I got a system up north that I put together and I have 10x100 watt panels in series-parallel configuration with 2x 5 panels in series and those wired in parallel. Therefore there are no power losses. The peak on my system is ~960 watts, with all panels facing south.

I get that I shouldn't see 9.6kW on peak but shouldn't the peak be at least 7.2kW when all 15 south-facing panels are at peak production? or is the voltage drop due to the west panels? I assume that they are on the string with 5 panels. and the two other strings are the south-facing ones.

or should the west panels really be on their own power inverter? and have equal strings on this inverter of 5 panels each with all facing south and then getting the 7.2kW peak?

Why is the AC output rating limited to 7.7 kW? is the inverter undersized for a 9.6 kW system?

It just does not make sense to me that they would design such a poor system. I am missing something? Should the MPPT controller make up for this?

Please help me understand what is going on here. The system that I build makes sense and performs as expected, their system does not make sense and does not perform as expected.

Thanks!

The number of panels on each MPPT isn't an issue as long as the total specification of the string is in-between the inverter specification PER MPPT.

"wouldn't you want even strings for no power losses?" -

Ideally even strings would be better but makes no difference in losses if each string is on its own MPPT. It only makes a difference if they were strung in parallel sharing a single MPPT.

"but shouldn't the peak be at least 7.2kW when all 15 south-facing panels are at peak production?" -

All depends on many things. Assuming you are in the northern hemisphere, the solar output for those times, the high level cloud cover, the angle of sun to the plane of the panels, the ambient temperatures etc etc etc.

"or is the voltage drop due to the west panels?" -

That depends if that string is on a separate MPPT. I would assume a competent installer would have put the south and west on different MPPTs else you would have a massive restriction. Suggest you check that each string is on its own MPPT.

"or should the west panels really be on their own power inverter?" -

Just needs to be on its own MPPT.

"and have equal strings on this inverter of 5 panels each with all facing south and then getting the 7.2kW peak?" -

You will unlikely be able to get peak unless the sun is providing equal max irradiation on both sides. If that does happen, would be rare and limited. As it is, it is most likely the west array will get higher power per panel on afternoons and less in mornings whereas the south will get even distribution but its peak afternoons would be less than the west side array.

"It just does not make sense to me that they would design such a poor system. I am missing something? Should the MPPT controller make up for this?" -

Unless you have more space to move the west array panels to the south roof, you are limited by space and make use of what's available. A smarter position for the smaller array would be to take into account the energy usage for the day and see whether you use more energy in the morning or afternoons so you can move those loads onto solar.

As far as system sizing is concerned, i think stating panel kWp as a system size is misleading. IMHO what really matters is the inverter output because that determines the usable energy available to the house loads. Having a standard ' one size fits all guide' of a 1.25 DC/AC ratio that most installers follow is a piss poor implementation/design of solar. Areas like in the south of the US where its hotter requires a much higher DC/AC ratio because of the massive drop in production during the hot months and/or large amounts of clouds/rainy days etc. Areas in the far north while being much colder have weaker solar radiation at times would IMHO also require adjustments in panels to inversion ratios. Knowing what works best through careful thought and experience is best.

[scrambler]

The string distribution does make sense.
Having the west panels on a different string than the South facing one is good. BUT as long as the current and voltage produce by these 5 panels actually is in the MPPT input range (have you checked that?)
That leaves 15 South facing panels that probably cannot fit onto one string so they are split evenly (almost as it is an uneven number of panel). here too you can double check if the Voltage and current potentially coming from each string is in the input range of the MPPT.

All that said, the 15 south facing panels represent 7.2kW of panels, so if unobstructed, when the sun starts getting higher and temperature are still not too hot, you should be able to get more that 5kW just for these.

The first thing to do would be to examine production graphs for each string separately (hopefully your inverter monitoring offers that possibility), to see how each string producing during clear sky sunny days, so you can see if they are all a bit low, or if may be one is seriously under performing pointing to a problem in one specific strings.

[oregon_phil]

I wonder if the OP is still engaged in this thread. No contact since original post on 7/31/23.