Solar installers giving me different info

[Watchdog]

Hello from Texas,

The more I read, the more I learn that I don't know a lot about solar setups. The installers I am shopping around here in San Antonio are like car salesmen (and I used to be one too)

I just want an efficient grid tied system for my home.

I like the Sunny Boy inverters from what I've read. And I have been offered two types of panels.

In the States, the best exposure is on the South side of a roof. The array I want (6 - 6.5 kW) would have panels on the South and East side of my roof. Now comes the tricky part...

The panels I have narrowed it down to are similar I believe. They are the Canadian Solar CS6P-250P and the Phono Solar - PS250P-20/U (Black/Black). In both systems, there will be 24 panels for a 6kW system or 26 for a 6.5kW system.

I am trying to decide what are the choices I have in an SMA inverter. My home will have an South and East array system. One set on the a South slope and the other on an East slope of the home. The West side is too small.

I have heard that if there are two different orientation of panels (South and East), that the panels have to be the same amount on each side (12 on the South and 12 on the East - total of 24 panels). If they are uneven, the inverter will be "unbalanced". Is this true? Again, I am getting two different answers from the companies I am talking to. One company says it's okay to put 11 panels on the South and 13 on the East side with the SMA SB6000TL-US-12 (240v) inverter. The other company says it has to be even set of panels on each side.

The SMA inverters they suggested were the SMA SB6000TL-US-22 and the SMA SB6000TL-US-12 (240v). I was leaning toward the SMA SB6000TL-US-12 (240v) but there is so much I don't understand which can affect production like...

how two arrays tie in and if they need to be "balanced",

if MPPT has anything to do it; does series or parallel have anything to do with my type of grid tied system

if I should take into account the start up voltage difference

I did a comparison of the inverters on a web page that gave me this...

http://www.solardesigntool.com/compare-inverters.html?submit=Compare+Inverters&inverters=453&inverters=492&_inverters=on&compare=true

I would appreciate any help you solar wizards can give me

[inetdog]

Hello Watchdog and welcome to Solar Panel Talk!

First of all: If the two arrays have equal numbers of panels and neither array has a partial shading issue at any time of the day where production is high (near solar noon), then you can just connect them in parallel to one MPPT input at the inverter.
But if the arrays are unequal numbers of panels or one of them gets partial shading, then you will need to connect each array to its own MPPT input to get the most possible power out of your panels.
One of the nice things about the East facing array is that it will extend the hours that your inverter is producing well. The bad thing is that if you are on a Time Of Use rate structure much of that output will be at a time of low per kWh rates. What is the weather like early in the morning when the sun is still low in the sky?

Your chosen SMA SB600TL-US has only one MPPT input, so I would say the first company salesman (or system designer) does not know what he is talking about (to put it politely.)
If that company is proposing an inverter which has two separate MPPT inputs (and there are quite a few of them), then the unbalanced arrays would be just fine. The SB 5000 TL-US does have two MPPT inputs and since the two arrays are producing their peak power at different times may even be large enough to handle the output of your 24 panels. The 5000 also has the SMA Secure Power feature which will give you up to 12A at 120V to a separate output for use when the grid is down.
Both families have "Optitrac" which SMA claims to give better performance with balanced arrays but different facing directions on one MPPT input.

Just looking at the SMA website I do not see the 6000-22 model with two MPPT inputs listed. Strange....

[silversaver]

SMA SB6000TL-US-22 is the new TL inverter with 2 MPPT.

[Watchdog]

Hello Watchdog and welcome to Solar Panel Talk!

First of all: If the two arrays have equal numbers of panels and neither array has a partial shading issue at any time of the day where production is high (near solar noon), then you can just connect them in parallel to one MPPT input at the inverter.
But if the arrays are unequal numbers of panels or one of them gets partial shading, then you will need to connect each array to its own MPPT input to get the most possible power out of your panels.
One of the nice things about the East facing array is that it will extend the hours that your inverter is producing well. The bad thing is that if you are on a Time Of Use rate structure much of that output will be at a time of low per kWh rates. What is the weather like early in the morning when the sun is still low in the sky?

Your chosen SMA SB600TL-US has only one MPPT input, so I would say the first company salesman (or system designer) does not know what he is talking about (to put it politely.)
If that company is proposing an inverter which has two separate MPPT inputs (and there are quite a few of them), then the unbalanced arrays would be just fine. The SB 5000 TL-US does have two MPPT inputs and since the two arrays are producing their peak power at different times may even be large enough to handle the output of your 24 panels. The 5000 also has the SMA Secure Power feature which will give you up to 12A at 120V to a separate output for use when the grid is down.
Both families have "Optitrac" which SMA claims to give better performance with balanced arrays but different facing directions on one MPPT input.

Just looking at the SMA website I do not see the 6000-22 model with two MPPT inputs listed. Strange....

Thanks for the answers. From what I have been reading, you confirm that it is better to have two MPPT with two different orientation when using a grid tied central inverter. I wasn't sure because some of the stuff I was reading was mentioning strings as an option, and the SB6000TL-US-12 has the capability of six string inputs for its one MPPT. So that is why it seemed muddy to me.

I was also leaning towards the SB6000TL-US-12 vs the SB-6000TL-US-22 because its Weighted Efficiency rating is 98% (208V)/98.5% (240V) vs 96.8% (208V) 97.1% (240V). And I liked that it had forced cooling (which raised the noise level to 46 dba, but it is outside) vs Natural Convection. And it is cheaper.

BUT... besides the limited MPPT, it also has a higher start-up voltage of 360V vs 150V. As for the backup power option of Secure Power Supply, I would like to have it and even pay for it, but it's not a make or break for me. I figure that since the power only cuts out a few days a year, and it usually happens when there is a big storm during the night... I wouldn't be generating much anyway. The only time it would be a big help is the occasional winter ice storms in San Antonio that happen about every 2-3 years that knock out power for a day, and then the sun comes out.

My priority is usable power generation and reliability. Everything else is secondary.

My roof is second story and gets no shade other than the East side when the sun is going down because the pitch is steep. The angles given from one proposal are 32° on the East side and 39° on the South side. He went up on the roof and took measurements.

You brought up the SB 5000 TL-US. I pulled a side by side comparisons here...

http://www.solardesigntool.com/compare-inverters.html?submit=Compare+Inverters&inverters=488&inverters=421&inverters=453&inverters=492&_inverters=on&compare=true

Even with the steep pitches on my roof; the East and South side will have full sun during the summer days at the same time during the middle of the day. So with a 6kW or 6.5kW system, won't a 5000 inverter limit the max potential DC output? I think I read that inverters can go over their rating by a certain percentage, but I am not sure if that applies when looking at the spec sheets. Also I keep seeing that the SB6000TL-US-12 has the highest Weighted Efficiency rating. BUT... what good is that if the inverter can't maximize it because of two arrays in a different orientation.

I looked up the info on the optitrac you mention, but once again, I am confused. It said here...

"Selecting the inverters
As described in “Shadow Management”, the choice of inverter also influences energy losses due to shade. Three points are to be observed when selecting the inverter:

• Inverters with a broad input voltage range can still adjust the optimal operating point despite shade and the resulting decline in MPP voltage.
• Using an inverter with a single-string control, a partially shaded PV array can operate optimally and avoid most losses.
• To keep losses (due to shade) to a minimum, it is necessary to use an inverter for partially shaded strings whose MPP tracking recognizes the existence of several operating points (e.g. OptiTrac Global Peak)."

So does that mean if there in only one MPPT with the ability for multiple strings and it has optitrac, there is no problem? The section is talking about shading, not two different arrays, so is that relevant?

The SB-6000TL-US-22 you were unable to find is here...

http://www.sma-america.com/en_US/products/grid-tied-inverters/sunny-boy/sunny-boy-with-sps-3000tl-us-3800tl-us-4000tl-us-5000tl-us-6000tl-us.html

I... AM... SO... CONFUSED.

[Watchdog]

Does this clear it up?

I found this on the SMA site. It seems to address the question of two arrays with different orientations on a single MPPT...

http://www.smainverted.com/2012/12/04/sunny-design-polystring-configuration/

Wait, won’t the two string voltages be different if they are facing opposite directions?

No! Luckily unbalanced string voltage is a completely separate issue. As the SMA study says, “The currents from the PV modules may vary significantly in the substrings over the course of the day. However, the voltage at MPP is practically identical.” This is because it only takes a small amount of light for the modules to achieve their rated DC Voltage.

Overall, it is not an optimum system design to have strings at significantly different orientations. However, there are instances where different orientations are the only option or the desired option. The results of the SMA study shows that you can operate a string inverter with a single MPP tracker with strings at different orientations. One test that was done in the study compares a single MPP tracking inverter with a dual MPP tracking inverter, both with due east and due west facing strings and the same site conditions. The results were only a 0.25% yield loss for the single MPP inverter.

So if the potential loss is only 0.25%, is that okay with the higher efficiency rating?

[silversaver]

you have 2 orientaion roofs, so 6000TL-US12 wont do. you need the 6000TL-US22 with dual MPPT. I'm running a 6000TL-US12 for my 7.1kW on the same roof (28 panels). Actually a 5000TL-US22 should have take care of your 6kW setup, but 6000TL-US22 comes with build in cooling fan as standard and you can add few more panels later if needed. Personally, I'll go with 26 to 28 panels setup with 6000-TL-US22 since adding panels later is a pain in the arse.... $$$$ too. Good luck

[bcroe]

I found this on the SMA site. It seems to address the question of two arrays with different orientations on a single MPPT...

http://www.smainverted.com/2012/12/04/sunny-design-polystring-configuration/

Wait, won’t the two string voltages be different if they are facing opposite directions?
No! Luckily unbalanced string voltage is a completely separate issue. As the SMA study says, “The currents from the PV modules may vary significantly in the substrings over the course of the day. However, the voltage at MPP is practically identical.” This is because it only takes a small amount of light for the modules to achieve their rated DC Voltage.
Overall, it is not an optimum system design to have strings at significantly different orientations. However, there are instances where different orientations are the only option or the desired option. The results of the SMA study shows that you can operate a string inverter with a single MPP tracker with strings at different orientations. One test that was done in the study compares a single MPP tracking inverter with a dual MPP tracking inverter, both with due east and due west facing strings and the same site conditions. The results were only a 0.25% yield loss for the single MPP inverter.

So if the potential loss is only 0.25%, is that okay with the higher efficiency rating?

Let me restate it. Panel operating voltage varies little on a given day, but the current is tied
to sunlight intensity. Strings of the same voltage (number of panels) may be tied in parallel;
if one is receiving less sun (orientation), it will continue to produce at a reduced current.

If a string has one or more panels shaded or producing less current, the output of the entire
string will be limited; all panels in a string should have similar orientation & lack of shade for
the string to produce.

The panel operating voltage will be about 20% less than its open circuit voltage. In addition
the voltage will increase in colder weather. The inverter & string length must keep the entire
voltage range within the inverter operating range.

With strings of different orientations, the peak currents of the strings will occur at different
times of day; therefore the inverter will probably never see the sum of the string peak
currents. Possibly this will allow a smaller inverter & AC wiring, or instead it could allow
additional panels of different orientations to keep the inverted at high power for more hours
of the day. The statement that different orientations are not optimum, depends on what
your objectives are. Bruce Roe

[Texican]

Same boat!!!

Watchdog, I am also here in SA and about where you are in the process. I got a couple of proposals for DC systems but my roof is broken up in three directions and I have panels facing in different directions on what appears like two strings. From what I have read on this forum is not going to be the most efficient system. Have you looked at micro inverters and optimizers? I am just reading up on this and it appears to be more efficient but costs a little more...


5755.jpeg

[Rdjntx]

The two of you should look at longhorn solar if you haven't already. They did my system on two roof faces (25 on S and 15 on W) and used two inverters. it has been up and running well for two years now. very professional in my experience.

[Ian S]

Even with the steep pitches on my roof; the East and South side will have full sun during the summer days at the same time during the middle of the day. So with a 6kW or 6.5kW system, won't a 5000 inverter limit the max potential DC output?

Maximum power is only produced when the sun's rays are perpendicular to the panels. Both the east and south sides will be illuminated at mid-day but sun's rays will considerably off the perpendicular for the east side. Likewise, when the sun's rays are closest to perpendicular on the east side, they will be quite far off perpendicular on the south facing array. So your observed combined power from the two arrays will never be close to nameplate because you'll never have the sun's rays perpendicular to all the panels at the same time. However, overall production will still be good because you'll have more in the AM than you would with just a south array.

[rdo]

Overall, it is not an optimum system design to have strings at significantly different orientations. However, there are instances where different orientations are the only option or the desired option. The results of the SMA study shows that you can operate a string inverter with a single MPP tracker with strings at different orientations. One test that was done in the study compares a single MPP tracking inverter with a dual MPP tracking inverter, both with due east and due west facing strings and the same site conditions. The results were only a 0.25% yield loss for the single MPP inverter.[/I]

So if the potential loss is only 0.25%, is that okay with the higher efficiency rating?

I've accidentally found an answer to something I was looking to establish, thanks.

I do have two equal strings, however, one is getting more shading in winter than another. I'm assuming this also applies if they're connected to the same MPPT or to the two separate ones (SMA 3600TL)?

I'm also wondering if you can connect two different sort of panels if you have two MPPT's...?

[Amy@altE]

Different manufacturers have different limitations to how unbalanced a dual MPPT inverter can be. I don't know off the top of my head for SMA. They do have an online string sizer, so not knowing all of your details, I entered a hypothetical system in NYC, with 12 250W panels S and 13 E (I couldn't remember your exact specs). I tried two 3000W TL-22, and one 5000TL-22, one 6000TL-12, and one 7000TL-22. It's projected annual output below:

two 3000W TL-22, 7145kwh
one 5000TL-22, 6936kwh
one 6000TL-22, 7283kwh
one 7000TL-22. 7223kwh

http://www.sunnydesignweb.com/sdweb/#/Home

Warning: It is not at all intuitive to use.

[bcroe]

I've accidentally found an answer to something I was looking to establish, thanks.

I do have two equal strings, however, one is getting more shading in winter than another. I'm assuming
this also applies if they're connected to the same MPPT or to the two separate ones (SMA 3600TL)?

I'm also wondering if you can connect two different sort of panels if you have two MPPT's...?

2 identical strings will produce close enough the same voltage when one has better sun; one MPPT
will work well. But the 2 strings with different shading WILL NOT, because the shaded sections
will have to go into bypass diode conduction, to maintain current at a lower overall voltage. That
will require separate MPPTs for best output. Bruce Roe

[rdo]

2 identical strings will produce close enough the same voltage when one has better sun; one MPPT
will work well. But the 2 strings with different shading WILL NOT, because the shaded sections
will have to go into bypass diode conduction, to maintain current at a lower overall voltage. That
will require separate MPPTs for best output. Bruce Roe

Thanks. Is there a way of establishing using the software and without removing the front plate of the inverter, whether the strings have been connected to the same MPPT or not?

[inetdog]

Thanks. Is there a way of establishing using the software and without removing the front plate of the inverter, whether the strings have been connected to the same MPPT or not?

With the strings both getting sun, measure the voltage on the + lead of both strings (back at the array or anywhere before the wires enter the inverter.
If the two voltages are equal, then both strings are connected to the same MPPT input.
If you want to be completely sure, you can cover one or more panels in one string and confirm whether the voltages are still equal.
If this SMA uses ungrounded array input, you may need to measure from - to + on each string to compare.