Best Ways to String Panels Together

[yblaser]

Hi All. Reading these threads about how people's systems are strung together it got me thinking about the system I have on order and what the optimum way to string the panels together is. The system we have on order is a Tesla 8kW, 24 panel system with panels pretty much facing every cardinal direction. We will have 8 panels facing east, 6 facing west, 8 facing south, and 2 facing north. How do you think these panels should be strung together.

I think I have a descent understanding of how this all works but would like some experts' opinions. I would guess you would want three strings in this case, 1 for the east, 1 for the west, and 1 for the south with the two north facing panels?

Thanks for the insight.

 

[FurryOne]

IMO, the North facing panels are bad position. Better to add them to the West section if possible, and use 3 strings.

 

[GJones]

Since you are still in the design phase - I agree with FurryOne. North facing arrays are lunacy unless there are no other options in terms of roof size and orientation. I went to my local building department (and fire department) to get variances during my design in order to maximize South and West orientations. It was worth the effort. I have a neighbor that was worried about the aesthetics of the panels on his roof and did not want to have any show from the street. He ended up with a completely northeast facing array (60degrees) and during the best production time of the day - his entire array is in a shadow. Based on CA utility rates - he is at max production to mitigate the lowest rates and is at a very severe disadvantage during the highest rates. He also decided not to do powerwalls. Darwinian failure...

 

[h2ofun]

Since you are still in the design phase - I agree with FurryOne. North facing arrays are lunacy unless there are no other options in terms of roof size and orientation. I went to my local building department (and fire department) to get variances during my design in order to maximize South and West orientations. It was worth the effort. I have a neighbor that was worried about the aesthetics of the panels on his roof and did not want to have any show from the street. He ended up with a completely northeast facing array (60degrees) and during the best production time of the day - his entire array is in a shadow. Based on CA utility rates - he is at max production to mitigate the lowest rates and is at a very severe disadvantage during the highest rates. He also decided not to do powerwalls. Darwinian failure...

Why are north, or even east lunacy? I just made a post with my estimates from PV watts. East was better than west. And north was not that bad.

 

[GJones]

It's not just the direction of the array, but also shading. Depending on the pitch of the roof, the entire array goes into shade. The Darwinian reference guy - his entire array is in full shade by about 4pm during the summer. 4 hours of production is penalized. The general rate structure here depending on time of use or EV rates crank up to maximum in the 2pm to 4pm region and continue well after dark. The time when the solar production has the most monetary value (and also has a chance for peak production) - it has already gone into decline.
The estimates are heavily impacted by direction, shading obstacles like trees and any other obstruction. In my example - the guys own roof is an obstacle. No two situations are going to be the same - my obviously perfunctory statements could be considered inflammatory in some situations - in this case - the guy had the opportunity to benefit from the insight a neighbor with an array in place for over a decade. One other thing - he also has a line of trees in his back yard shading his array in the morning.

 

[Patrick66]

Need to establish what your roof pitch is first.

 

[h2ofun]

It's not just the direction of the array, but also shading. Depending on the pitch of the roof, the entire array goes into shade. The Darwinian reference guy - his entire array is in full shade by about 4pm during the summer. 4 hours of production is penalized. The general rate structure here depending on time of use or EV rates crank up to maximum in the 2pm to 4pm region and continue well after dark. The time when the solar production has the most monetary value (and also has a chance for peak production) - it has already gone into decline.
The estimates are heavily impacted by direction, shading obstacles like trees and any other obstruction. In my example - the guys own roof is an obstacle. No two situations are going to be the same - my obviously perfunctory statements could be considered inflammatory in some situations - in this case - the guy had the opportunity to benefit from the insight a neighbor with an array in place for over a decade. One other thing - he also has a line of trees in his back yard shading his array in the morning.

I actually have less shading on my north sides than my south because of my neighbors trees.

 

[h2ofun]

Need to establish what your roof pitch is first.

Mine seem to be either 3/12 or my north is 4/12

 

[yblaser]

Thanks for all the replies.

The roof pitch is 15°.

They originally gave me this layout (south east portion of the house has a second story):

but after the onsite inspection and just as they started to file for the permits they updated the layout. The updated layout had four panels facing north and only six facing south. I had them move two of the panels on the north to the south resulting in the layout below.

It looks like one of the main differences that caused the layout to change is increased setback from the ridge line but I'm only guessing based on the changes.

The only place to move the north facing panels would be the west facing first floor roof, but that area gets shaded from the second story late into the morning and there is a tree to the west that will shade the area in the late afternoon/evening.

At this point I don't know if I want to restart the clock on the design to move just two panels. I figure in the summer the north and south panels will have similar productions at similar times of day so they should be on the same string. In the winter I assume the north panels won't produce much and may just be "off" a large portion of the day. Does that negatively impact the other panels on the string?

 

[charlesj]

May want to figure out how much those two north facing panels will generate in a year and if that is cost effective for those two.
May want to just consider dropping them?

 

[Patrick66]

Might want to invest in moving some vent pipes to see if that gives you a better layout. But, at just 15 degrees slope it isn’t the end of the world. I wouldn’t put the north facing panels on a south facing string; you will kill your winter production.

 

[yblaser]

I wouldn’t put the north facing panels on a south facing string; you will kill your winter production.

Thanks. Can you expand on this a little? Will it just kill production from those two panels, which I am okay with, or does it affect production of the rest of the string, which I am not okay with?

 

[yblaser]

Okay, after educating myself and looking at some VI curves I see the issue with adding the two north panels to the south string. It's not that the north panels would operate sub optimally but rather they would be bypassed most of the year when there is uneven illumination between the two sides.

I'm wondering what Tesla is going to do then since two panels are not enough for a single string.

Since their second design had 4 panels and the north side and I asked them to move two panels from the north to the south side will they just come back and say that it can't be done?

 

[dareed1]

According to the specs shown on a photo in arnolddeleon's thread the minimum MPPT voltage for the Tesla inverter is 60V. So in principle 2 panels are enough since each panel makes >30V even in shade (from diffuse illumination). It isn't clear how efficiently the Tesla inverter will work with a 2 panel string when illuminated with less than say 200W. I do know that my Delta M8-TL-US will operate at 100W, but that is at higher voltage (at least 3 panels with the rest shaded).

I'm not necessarily recommending a 2 panel string, but it isn't obviously wrong, and almost surely better than incorporating the 2 north facing panels into another string. They would probably never get as much insolation as the other arrays, and thus would always be bypassed.

 

[yblaser]

According to the specs shown on a photo in arnolddeleon's thread the minimum MPPT voltage for the Tesla inverter is 60V. So in principle 2 panels are enough since each panel makes >30V even in shade (from diffuse illumination). It isn't clear how efficiently the Tesla inverter will work with a 2 panel string when illuminated with less than say 200W. I do know that my Delta M8-TL-US will operate at 100W, but that is at higher voltage (at least 3 panels with the rest shaded).

I'm not necessarily recommending a 2 panel string, but it isn't obviously wrong, and almost surely better than incorporating the 2 north facing panels into another string. They would probably never get as much insolation as the other arrays, and thus would always be bypassed.

Thanks I did look up that thread earlier as I remember someone had posted that picture with the input specs. I wasn't sure what the panel voltage was in various conditions outside what it says in the spec sheet. Good to know that it is over 30V even in diffuse light. I guess I'll see what Tesla ends up doing.

I assume once Tesla is ready to submit the permits I can ask for the design documents which will show how everything is wired up. Is that correct?

BTW, I grew up in your city..

 

[Edev0]

Add 2 more north panels for a total of 4 on that area of the roof

 

[yblaser]

Just for an update.... The system is not installed yet as predictably there has been some delays. I intended to leave the design as is but I went ahead and requested the two north facing panels to be moved to the first story west facing roof since the redesign was not going to impact the total schedule as there were other longer duration issues.

Anyway, I finally requested the full design package and was quite surprised on how they choose to wire up the panels. They kept the 2 panels I requested to move to the west roof on their own 2 panel string. For the two south facing panel sets (4 panels each) they also kept on separate strings. Interestingly they say my garage roof is 1° off the rest of the house which faces the cardinal directions. Maybe that is the reason but seems quite small to me. As a result of these choices the number of strings exceeds what the 7.6 kW inverter can handle and as a result they added a second 3.8 kW inverter. Well I guess I'm never going to see any clipping.

For reference:

MP1: South (181°) facing garage
MP4: South facing 1st story
MP6: West facing 1st story
MP5: West facing 2nd story
MP3: East facing 2nd story

 

[zƬesla]

Are two panels enough to energize a string?

 

[yblaser]

Are two panels enough to energize a string?

It is according to the Tesla inverter specifications but if there is any partial shading on either panel it will knock out the whole string.