Question regarding string wiring (parallel vs series) of my panel system

[Sudsington]

Hi all, I'm looking for some insight on whether my panels are currently wired optimally for my situation. I don't have a background as an electrician or anything, so apologies in advance if I make any incorrect assumptions.

I had my 18-panel (340W Qcells) system installed in September 2021. My install includes 8 panels on my main roof (4 south-facing, 4 east-facing) and 10 panels on my detached garage (5 south-facing, 5 north-facing). For about 4-5 months of the year (late fall through early spring) I have some shading on some or all of my south-facing garage panels from 10am-12pm. No shading on any of the other 13 panels. So I asked for the installers to keep that in mind when they set up strings. Recently, I had a DC arc fault 36 error show up on my inverter, and my service tech rewired the inverter and changed the strings from 2 strings wired in parallel (jumpers on Strings 1 and 2) to 2 strings wired in series (no jumpers).

I want to know if there are any drawbacks to the current wiring set-up, and if I need to push a fix so the two sets of panels can each be wired in parallel again. I tried to include relevant info, but let me know if there's any other info that would help. Any insight is much appreciated, as I don't want to leave an issue unaddressed, but I also don't want to bother the technician if the current wiring fits my needs.



Layout on roofs. Bottom of image is South, so 4 panels on main roof are S, 4 panels are E. 5 panels on garage roof are S and 5 are N. Partial shading on garage S roof from 10am-12pm).

Wiring plan:

Initial inverter wiring when installed (parallel wiring on Strings 1 and 2):

Inverter wiring after tech visit due to DC arc fault 36 (series wiring on Strings 1 and 2):

Production on cloudless days (similar temperatures): prior to arc fault (parallel wiring) vs after repair/rewiring (series wiring):

 

[arnolddeleon]

Both of your configurations are basically the same.

Each array is getting their own MPPT (Maximum Power Point Tracking) input in the configuration. This means each array can be at a different MPPT voltage.

When they are jumped together the MPPT inputs work together but they have to be at the same voltage. This is useful when you have a large array (or several arrays) oriented in the same way that exceed the capacity of one input. Your original configuration had inputs 1/2 paired and 3/4 paired.

So your original configuration was using all the of the inputs and the second configuration is using only half (1 and 2 which are now split). So its possible that something went wrong with input 3 or 4 and the new configuration avoids it. This is non-issue as long as inputs can handle the output of your solar arrays.

So says the random guy on the Internet

 

[yblaser]

It doesn't look like they changed how your panels are wired, series vs parallel, since the voltage of the string is largely unchanged. Rather they just removed the jumpers so that the parallel string are wired into a single inverter input verses shared between two inverter inputs. I don't know why they did this but maybe one of the inverter inputs was bad causing the arc fault and they did this instead of replacing the inverter? Who knows?

If my guess on what they did is correct I would be concerned that as we approach peak solar production your parallel strings could exceed the current rating for a single inverter input near solar noon. I have no idea what the inverter does in that case but in the best case it would clip production.

Optimally they should have run four returns to the inverter, one for each string. You should never have panels with different orientations on the same series string. Parallel strings can work but especially with shading they're less than ideal.

 

[Sudsington]

Both of your configurations are basically the same.

Each array is getting their own MPPT (Maximum Power Point Tracking) input in the configuration. This means each array can be at a different MPPT voltage.

When they are jumped together the MPPT inputs work together but they have to be at the same voltage. This is useful when you have a large array (or several arrays) oriented in the same way that exceed the capacity of one input. Your original configuration had inputs 1/2 paired and 3/4 paired.

So your original configuration was using all the of the inputs and the second configuration is using only half (1 and 2 which are now split). So its possible that something went wrong with input 3 or 4 and the new configuration avoids it. This is non-issue as long as inputs can handle the output of your solar arrays.

So says the random guy on the Internet

Thanks for the explanation, that definitely helps me the potential benefits of how it was jumpered before, as opposed to the current wiring. I'm hoping that the current wiring allows for full production on both arrays. I checked my production stats from May/June of last year, and I peaked at about 4.8 kW (~36 kWh generated in one day). I'm hoping that the two inputs are capable of handling that, but I don't know how I would calculate it to confirm.

 

[Sudsington]

It doesn't look like they changed how your panels are wired, series vs parallel, since the voltage of the string is largely unchanged. Rather they just removed the jumpers so that the parallel string are wired into a single inverter input verses shared between two inverter inputs. I don't know why they did this but maybe one of the inverter inputs was bad causing the arc fault and they did this instead of replacing the inverter? Who knows?

If my guess on what they did is correct I would be concerned that as we approach peak solar production your parallel strings could exceed the current rating for a single inverter input near solar noon. I have no idea what the inverter does in that case but in the best case it would clip production.

Optimally they should have run four returns to the inverter, one for each string. You should never have panels with different orientations on the same series string. Parallel strings can work but especially with shading they're less than ideal.

That's correct, I should have clarified that in the post. It seems that the 2 sets of 4 panels on my main roof are wired parallel (according to the wiring schematics and what the service tech told me), and same for the 2 sets of 5 panels on my garage roof. The service tech only changed the wiring in the inverter, and did not rewire my panels. I'm somewhat disappointed that the original installers wired those panels facing different orientations (South and East; North and South) in parallel, but I'm not certain how much that affects my annual production as compared to a series-wired configuration. I only have shading issues for an hour or two during the winter (improves significantly in April), so I don't know how much I'm really losing.

Is there a way to estimate if the inputs will be maxed out if I know how much I produce at my most optimal time of year? My max kW produced peaked around 4.8 kW in May/June of last year. Granted, I don't know how that was split between the two arrays (main roof and garage roof).

 

[arnolddeleon]

That's correct, I should have clarified that in the post. It seems that the 2 sets of 4 panels on my main roof are wired parallel (according to the wiring schematics and what the service tech told me), and same for the 2 sets of 5 panels on my garage roof. The service tech only changed the wiring in the inverter, and did not rewire my panels. I'm somewhat disappointed that the original installers wired those panels facing different orientations (South and East; North and South) in parallel, but I'm not certain how much that affects my annual production as compared to a series-wired configuration. I only have shading issues for an hour or two during the winter (improves significantly in April), so I don't know how much I'm really losing.

Is there a way to estimate if the inputs will be maxed out if I know how much I produce at my most optimal time of year? My max kW produced peaked around 4.8 kW in May/June of last year. Granted, I don't know how that was split between the two arrays (main roof and garage roof).

It seems that Tesla will often wire arrays in different orientations together. I suspect it ends up not mattering much. It actually ends up using the inverter more efficiently, instead of a narrow peak you get two curves shifted from each other that are overlayed.

One of the things I've learned to appreciate that "good enough" often is. Could you theoretically have more production if each array was on a separate input. On paper yes but in the real world the difference may not pay for the extra copper and labor to run down the additional lines. On the other hand the system might actually produce a little more power in the configuration that Tesla installed because the inverter inputs is running at higher power outputs more of time. Power electronics efficiencies vary, usually there is range where efficiency is higher. At the low and absolute highest it is often lower. I'll summon @nwdiver who has shared much wisdom on the idea of cost-efficient PV installs.

I suspect because of the different orientations of your arrays you likely won't hit clipping. And even if it did clip, you would be hard pressed to measure to the difference in energy production. You'll have data soon enough.

 

[Sudsington]

It seems that Tesla will often wire arrays in different orientations together. I suspect it ends up not mattering much. It actually ends up using the inverter more efficiently, instead of a narrow peak you get two curves shifted from each other that are overlayed.

One of the things I've learned to appreciate that "good enough" often is. Could you theoretically have more production if each array was on a separate input. On paper yes but in the real world the difference may not pay for the extra copper and labor to run down the additional lines. On the other hand the system might actually produce a little more power in the configuration that Tesla installed because the inverter inputs is running at higher power outputs more of time. Power electronics efficiencies vary, usually there is range where efficiency is higher. At the low and absolute highest it is often lower. I'll summon @nwdiver who has shared much wisdom on the idea of cost-efficient PV installs.

I suspect because of the different orientations of your arrays you likely won't hit clipping. And even if it did clip, you would be hard pressed to measure to the difference in energy production. You'll have data soon enough.

Thanks for this perspective. After reading a little more about panels wired in series vs parallel (posted in another thread), I'm not as worried about my North and South facing garage array (which are on a 4/12 pitch). I'll keep an eye on my production and compare it to last year, to see if the lack of jumpers in the inverter is affecting the max allowable input from my main roof (South and East facing panels) array.

My concern now is that whatever issue caused the tech to remove the jumpers on inputs 3 and 4 basically makes the inverter a ticking time bomb. When it fails, then I'll have to wait for an inverter to come into stock. I wish they would replace it while it's still functioning on inputs 1 and 2.

 

[wwu123]

Thanks for this perspective. After reading a little more about panels wired in series vs parallel (posted in another thread), I'm not as worried about my North and South facing garage array (which are on a 4/12 pitch). I'll keep an eye on my production and compare it to last year, to see if the lack of jumpers in the inverter is affecting the max allowable input from my main roof (South and East facing panels) array.

My concern now is that whatever issue caused the tech to remove the jumpers on inputs 3 and 4 basically makes the inverter a ticking time bomb. When it fails, then I'll have to wait for an inverter to come into stock. I wish they would replace it while it's still functioning on inputs 1 and 2.

I don't want to be a Debbie Downer here, but I think tying together North and South can be very different, geometrically speaking, than tying together East and West as in the article. I used to do spherical trigonometry on a daily basis for a former life, but my aging brain is too addled to bother to do it now. But general simple visualization, on a 4/12 pitch, each plane is tilted 18.5 deg in an opposing direction, so the two planes are 37 deg apart. On East-West pair, that 37 deg different is most pronounced towards dawn and dusk, there will be a time when the sun angles are closer to that 37 deg difference, but at solar noon, they will be exactly the same sun angle, and slightly off solar noon with the sun high in the sky, the actual angular difference will be different but fairly small. So the panel angles and insolation are most closely matched when the insolation mid-day is highest (i.e. when it matters), and angles most different when sun is low in the sky and insolation is low to begin with.

But for the North-South pair, it's the opposite, the 37 deg difference is most pronounced at solar noon when insolation is highest, and least pronounced towards dawn and dusk during lower insolation. Cosine of 37 deg says North sees about 80% of the insolation of the South panels at solar noon - not huge, but not zero difference.

That all said, those are strictly the differences in insolation, which are already decided by the roof orientations, nothing to do with MPPT inputs. Whereas the real question is how tying together the MPPT tracking moves for the differing insolation reduces one or both orientations too much away from their individual MPPT optimized points. I understand very little about how MPPT curves work for two panels of different orientations, so it may again reduce the power very little for North-South as well.

The other arguments, like how different panel temps mitigate slightly the off-axis orientation from the one closer to the sun, seem to definitely make sense for both East-West and North-South pairs....

 

[Sudsington]

I don't want to be a Debbie Downer here, but I think tying together North and South can be very different, geometrically speaking, than tying together East and West as in the article. I used to do spherical trigonometry on a daily basis for a former life, but my aging brain is too addled to bother to do it now. But general simple visualization, on a 4/12 pitch, each plane is tilted 18.5 deg in an opposing direction, so the two planes are 37 deg apart. On East-West pair, that 37 deg different is most pronounced towards dawn and dusk, there will be a time when the sun angles are closer to that 37 deg difference, but at solar noon, they will be exactly the same sun angle, and slightly off solar noon with the sun high in the sky, the actual angular difference will be different but fairly small. So the panel angles and insolation are most closely matched when the insolation mid-day is highest (i.e. when it matters), and angles most different when sun is low in the sky and insolation is low to begin with.

But for the North-South pair, it's the opposite, the 37 deg difference is most pronounced at solar noon when insolation is highest, and least pronounced towards dawn and dusk during lower insolation. Cosine of 37 deg says North sees about 80% of the insolation of the South panels at solar noon - not huge, but not zero difference.

That all said, those are strictly the differences in insolation, which are already decided by the roof orientations, nothing to do with MPPT inputs. Whereas the real question is how tying together the MPPT tracking moves for the differing insolation reduces one or both orientations too much away from their individual MPPT optimized points. I understand very little about how MPPT curves work for two panels of different orientations, so it may again reduce the power very little for North-South as well.

The other arguments, like how different panel temps mitigate slightly the off-axis orientation from the one closer to the sun, seem to definitely make sense for both East-West and North-South pairs....

No worries, that's a fair point! I should have elaborated that what I took from that article is that there are several factors for determining production loss when tying panel arrays of different azimuths together. My hope is that the losses are minimal (similar to minor clipping due to inverter sizing), and didn't warrant the extra material and labor cost. But I wouldn't be able to know for sure unless they agreed to re-wire the North/South panels to their own MPPTs.

 

[CrazyRabbit]

They did not wire your strings per the engineering. Result is they have drastically decreased your production. Have Tesla come back out and wire the strings to the engineering you BOTH agreed to.
Start arbitration NOW.
just watch, they will revise your engineering to what they have done. Make sure you save your engineering now.

 

[Sudsington]

Just an update regarding my situation. In June I was hitting peak power of ~3.8 kW (compared to 4.8 kW last June) and peak daily energy production of ~30 kWh (compared to 36 kWh last June). My case was escalated to Tier 2 and they found that my inverter was triggering an arc fault unnecessarily after production surges (cloud lensing, for example). They pushed a software fix of some type and sent the tech out to wire my system as it was originally installed. After that, my peaks and daily production have returned to the patterns seen last year. Thanks all for all the help and advice provided.

 

[BGbreeder]

I am wondering if a series Parallel set up might be better for my solar setup. I have 4 100 watt panels and a 40 amp MPPT controller. I need to run 20 feet of aw10 wire from the panels to the controller and 2 feet of aw10 to the batteries. The series would double the voltage and the parallel would double the amps to charge the batteries. or am I all wet? Otherwise I would go Series only. Any help or advice would be appreciated.

What are the specifications on your controller, and what are you trying to achieve?

 

[Matt-FL]

My experience with Tesla - the designer takes into account shading and facing direction when creating the design. However, when the installers arrive, they many times alter the design to take into account a simpler install.

In my case, the designer had taken into account some shading detail I had shared, and had split my east and west facing panels into two strings each. Installers decided that wasn't necessary and wired the 11 panels on each side into a single string (1 string east, 1 string west).

For the main southern array (29 panels), the designer had split half to one inverter along with east 11 panels,, and the other half to the second inverter with the west 11 panels. Installers, once again, decided to make this into two strings connected parallel into a single inverter (easier to install). So I have 29 panels on one inverter, and 22 on the second inverter. One inverter is cranking all day, the other is mostly just cruising.

I should mention, I do have a second string on the southern array inverter, but not sure what it comprises. I will make the assumption that they couldn't put an odd number of panels into parallel, so they broke off some panels into a single string.

At the time, I didn't know any better, but thanks to those 'random guy on the internet' experts , I know better now. That doesn't help solve my situation, but at least I can provide my experience with others so they can look out for it.