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Solar roof wiring?

Zorg

Active Member
Oct 24, 2017
1,374
1,219
Fremont, ca
I'm curious. How are solar roofs wired? I imagine that the tiles are connected in series to form multiple strings? If so, does partial shading kill the string production?

Does anyone know?
 

nwdiver

Well-Known Member
Feb 17, 2013
7,578
9,669
United States
I'm curious. How are solar roofs wired? I imagine that the tiles are connected in series to form multiple strings? If so, does partial shading kill the string production?

Does anyone know?

Sure sure how they're wired but they're going to be required to incorporate bypass diodes. Bypass diodes don't just allow unshaded portions to continue producing energy... they also prevent shaded sections from producing fires.
 

nwdiver

Well-Known Member
Feb 17, 2013
7,578
9,669
United States
You can watch YouTube videos of installations. Here’s one that shows linking the wires at 2:50:

I'm always a little amused at how often voltage is confused with power. The voltage should be higher on a cloudy day since voltage is proportional to temperature so you're gonna have higher voltage on a cool cloudy day than a hot sunny one.

In the video they say each shingle is ~13v so a string would be roughly ~40 shingles? I think it should be very shade tolerate so long as the inverter is smart enough to find the right point on the MPPT curve.
 

trautmane2

Member
Jul 13, 2019
203
138
Leesburg, VA
so a string would be roughly ~40 shingles?
My plans state:

Screen Shot 2021-03-11 at 7.34.51 PM.png
 

nwdiver

Well-Known Member
Feb 17, 2013
7,578
9,669
United States
Mine was very similar, I have a super string with two 35 tile strings in parallel as PV1, then two smaller 24 tile strings for PV2/ PV3 on a 3 string system.

How many kW? Was this before the Tesla inverters? It's a little surprising they did parallel strings. If there's no shade I guess that should be ok but if one string gets shaded production of the unshaded panels goes down. I have an old SMA inverter with 3 parallel strings since it only has 1 MPPT tracker. In the evening 3 panels get shaded in a 14 panel string. Basically kills the other 11 panels even though they're still in ~full sun. Happens around 5pm only in the summer so I'm not losing that much production.
 

trautmane2

Member
Jul 13, 2019
203
138
Leesburg, VA
My solar roof is a 9.94 kW system with 3 paired/parallel? strings connected to a Delta M8 inverter.

The PV1 pair has: 36 south roof tiles + 36 south roof tiles.
The PV2 pair has: 28 south roof tiles+ 28 east roof tiles.
The PV3 pair has: 21 west roof tiles + 21 west roof tiles.

Here are production values for a recent day that I pulled from the M Professional app:
Screen Shot 2021-03-12 at 3.30.36 PM.png


I think the little bump up after 4pm is when my south facing roof is shaded but my west facing roof gets a small burst sunshine that sneaks by my neighbor's house as the sun sets. @nwdiver 's comment about shading has me wondering about my PV2 strings which are split - one east and one south.
 

wwhitney

Member
Nov 2, 2017
765
919
Berkeley, CA
The voltage should be higher on a cloudy day since voltage is proportional to temperature so you're gonna have higher voltage on a cool cloudy day than a hot sunny one.
parallel strings . . .if one string gets shaded production of the unshaded panels goes down.

Let's see if I understand how the two above statements are consistent: V_MPP depends more on temperature than on % irradiance; so on a cool cloudy day, V_MPP will be lower than on a cool sunny day, but still higher than a hot sunny day.

When strings are in parallel, then V_string has to match for the two strings; but when one is shaded, some of its bypass diodes start conducting, so its V_string goes down. That drags down the V_string of the other unshaded string, away from its V_MPP.

Is that right?

Cheers, Wayne
 

nwdiver

Well-Known Member
Feb 17, 2013
7,578
9,669
United States
@nwdiver 's comment about shading has me wondering about my PV2 strings which are split - one east and one south.
Shouldn't be toooo bad.... unless one string gets shade and the other doesn't. The current through each string can vary but the voltages should be the ~same. Voltage changes with temperature and current with sunlight. So there will be a slight mismatch since the one in more sun would also be a little warmer but I don't think it would be significant.
 

nwdiver

Well-Known Member
Feb 17, 2013
7,578
9,669
United States
Is that right?

Cheers, Wayne

Mostly; V_MPP is inverse to temperature so it's higher on a cool cloudy day and lower on a hot sunny day.

Yeah... parallel strings are terrible. That's why the new Tesla inverters have 2 or 4 MPP channels. Parallel strings should be avoided at all cost.
 
Mar 1, 2021
117
31
San Diego, CA
How many kW? Was this before the Tesla inverters? It's a little surprising they did parallel strings. If there's no shade I guess that should be ok but if one string gets shaded production of the unshaded panels goes down. I have an old SMA inverter with 3 parallel strings since it only has 1 MPPT tracker. In the evening 3 panels get shaded in a 14 panel string. Basically kills the other 11 panels even though they're still in ~full sun. Happens around 5pm only in the summer so I'm not losing that much production.
6.8kW total.

I didn't realize shade on one parallel string messed up the other. How much of an impact is it? Just an efficiency decrease or a total reduction?

They didn't send me the design before installation (or even during installation) so I didn't know until after I got PTO. I guess there's not much I can do about it now, but shade in my lower roof is definitely going to pull down the string...
 
Mar 1, 2021
117
31
San Diego, CA
How many kW? Was this before the Tesla inverters? It's a little surprising they did parallel strings. If there's no shade I guess that should be ok but if one string gets shaded production of the unshaded panels goes down. I have an old SMA inverter with 3 parallel strings since it only has 1 MPPT tracker. In the evening 3 panels get shaded in a 14 panel string. Basically kills the other 11 panels even though they're still in ~full sun. Happens around 5pm only in the summer so I'm not losing that much production.
6.8kW total.

I didn't realize shade on one parallel string messed up the other. How much of an impact is it? Just an efficiency decrease or a total reduction?

They didn't send me the design before installation (or even during installation) so I didn't know until after I got PTO. I guess there's not much I can do about it now, but shade in my lower roof is definitely going to pull down the string...
My solar roof is a 9.94 kW system with 3 paired/parallel? strings connected to a Delta M8 inverter.

The PV1 pair has: 36 south roof tiles + 36 south roof tiles.
The PV2 pair has: 28 south roof tiles+ 28 east roof tiles.
The PV3 pair has: 21 west roof tiles + 21 west roof tiles.

Here are production values for a recent day that I pulled from the M Professional app:
View attachment 643910

I think the little bump up after 4pm is when my south facing roof is shaded but my west facing roof gets a small burst sunshine that sneaks by my neighbor's house as the sun sets. @nwdiver 's comment about shading has me wondering about my PV2 strings which are split - one east and one south.
Interesting that they run in parallel, I guess they have to do that to stay under max input voltage?
 

nwdiver

Well-Known Member
Feb 17, 2013
7,578
9,669
United States
6.8kW total.

I didn't realize shade on one parallel string messed up the other. How much of an impact is it? Just an efficiency decrease or a total reduction?

Depends on the situation but it can be somewhat dramatic. I think worst case is if only one string is partially in shade to the extent that the unshaded portion is unable to achieve sufficient voltage to allow any current flow. Basically if 30% of one string is in shade the other ~70% that's unshaded could stop producing if the parallel string is also unshaded. Voltage in parallel strings needs to be ~equal. But if there's no shade you should be fine.
 
Mar 1, 2021
117
31
San Diego, CA
Depends on the situation but it can be somewhat dramatic. I think worst case is if only one string is partially in shade to the extent that the unshaded portion is unable to achieve sufficient voltage to allow any current flow. Basically if 30% of one string is in shade the other ~70% that's unshaded could stop producing if the parallel string is also unshaded. Voltage in parallel strings needs to be ~equal. But if there's no shade you should be fine.
There is a lot of shade on my lower roof.

I've been seeing things like this where production jumps up at 8 and 2, with dips at 9 and 1, due to shade on the lower roof from my chimney in the AM, and the upper roof in the PM.
 

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h2ofun

Active Member
Aug 11, 2020
1,561
250
auburn, ca
There is a lot of shade on my lower roof.

I've been seeing things like this where production jumps up at 8 and 2, with dips at 9 and 1, due to shade on the lower roof from my chimney in the AM, and the upper roof in the PM.
I have total shade on my west panels from my garage until like noon. Just the way it is for me :( Only way to fix is add more panels :(
 

nwdiver

Well-Known Member
Feb 17, 2013
7,578
9,669
United States
There is a lot of shade on my lower roof.

I've been seeing things like this where production jumps up at 8 and 2, with dips at 9 and 1, due to shade on the lower roof from my chimney in the AM, and the upper roof in the PM.

Certainly looks like that's what's happening. The entire string is starting and stopping production. I get a similar effect around 5 in the evening in the summer.

You mentioned having 3 strings. What kind of inverter do you have?
 
Mar 1, 2021
117
31
San Diego, CA
Certainly looks like that's what's happening. The entire string is starting and stopping production. I get a similar effect around 5 in the evening in the summer.

You mentioned having 3 strings. What kind of inverter do you have?
That's what I was afraid of.

I have a delta M8, 3 strings, I think they probably should have wired up my 2x23 tile strings in parallel to 1x MPPT, they are both on the garage roof very close to each other with no shading issues. Then the lower roof 35 tiles could be on its own MPPT, and the top roof 35 tiles wouldn't be affected by the shading...
 

nwdiver

Well-Known Member
Feb 17, 2013
7,578
9,669
United States
That's what I was afraid of.

I have a delta M8, 3 strings, I think they probably should have wired up my 2x23 tile strings in parallel to 1x MPPT, they are both on the garage roof very close to each other with no shading issues. Then the lower roof 35 tiles could be on its own MPPT, and the top roof 35 tiles wouldn't be affected by the shading...

Does sound like that would have been better. Wonder where the strings are joined. If it's on the roof that would be a PITA to rewire but they might have run them separately into the inverters combiner box. If that's the case it might only take ~20 minutes to fix.

It's not so much that the string in shade is effecting the unshaded string. Essentially 2 different power points are being created since one string is effectively shorter than the other if one is in shade. So there's a 'M'PP at ~300v and one and ~400v. The inverter has to determine which one yields more power. At the higher voltage the unshaded string is basically producing at it's MPP and the shaded string is at ~0. At the lower voltage it's compromising and neither string is happy but overall power could be higher than the one unshaded string alone. Either way.... unshaded panels are losing power because of shaded panels... which is why parallel strings suck.
 

jboy210

Supporting Member
Dec 2, 2016
4,806
2,961
Northern California
Does sound like that would have been better. Wonder where the strings are joined. If it's on the roof that would be a PITA to rewire but they might have run them separately into the inverters combiner box. If that's the case it might only take ~20 minutes to fix.

It's not so much that the string in shade is effecting the unshaded string. Essentially 2 different power points are being created since one string is effectively shorter than the other if one is in shade. So there's a 'M'PP at ~300v and one and ~400v. The inverter has to determine which one yields more power. At the higher voltage the unshaded string is basically producing at it's MPP and the shaded string is at ~0. At the lower voltage it's compromising and neither string is happy but overall power could be higher than the one unshaded string alone. Either way.... unshaded panels are losing power because of shaded panels... which is why parallel strings suck.
My strings run down to boxes in the attic and from there down to the inverters.
 
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