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6 Parallel vs. 12 Series Solar panels


Atti2de
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Hello all!

What information is out there to choose from either one of the following options.

I have 12 solar panels that are all mounted on the same roof surface without any trees or other obstructions (or any other shadow creation devices). I want to connect this to my Goodwe inverter which has 2 MPPT chargers built-in.

A solar installer mentioned that it would be better to run 2 input strings to the inverter and utilise the 2 charger inputs. When asking why, I didn't really get a clear answer. 

Who here can provide a proper answer on this? 1 string of 12 (higher voltage) or 2 strings of 6?

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30 minutes ago, Atti2de said:

When asking why, I didn't really get a clear answer.

Has to do with independent tracking. If one panel in the array is shaded partially it affects the entire array. If you have two arrays on two MPPTs, then it only affects half your panels. If they all point in the same direction with no obstructions, there might be little benefit in using a single MPPT.

Other things to look at is the rating of the MPPT, voltage ranges and so on. 12 panels puts you somewhere between 450V and 600V depending on panel size. Might mean it starts charging earlier in the morning, but might be less efficient than aiming more for the middle. Also, DC voltages that high really scare me.

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Understood. Using the datasheet I can see that the max Vout from 6 would be about 360V. I think he must have thought that if 12 were connected that we would be over the inverters' max input.

I guess I can always series them just before the inverter (after checking Vout1 + Vout2). Totally agree with high DC voltages! It can be a nightmare.

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On 2018/08/17 at 7:03 AM, Atti2de said:

Understood. Using the datasheet I can see that the max Vout from 6 would be about 360V. I think he must have thought that if 12 were connected that we would be over the inverters' max input.

I guess I can always series them just before the inverter (after checking Vout1 + Vout2). Totally agree with high DC voltages! It can be a nightmare.

That's my first guess. Which panels do you have?

 

As @plonkster said, unless you have some shading issues, or the panels are on different facing roofs, there probably wouldn't be much difference. So I'm guessing 12 might be at the max or close to the max of the MPPT's voltage rating. 

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Hi ho Silver!

i am using the Canadian solar 275W panels. As mentioned, the total of input voltage should be around 360VDC. I managed to finish the installation today, but didn’t have full sun to check max voltage. I did get 180VDC on each string. Just wondering if anyone had specific experiences with a setup where one string would be preferred. At the end of the day it is all about the wattage.

 

 

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13 minutes ago, Atti2de said:

Hi ho Silver!

i am using the Canadian solar 275W panels. As mentioned, the total of input voltage should be around 360VDC. I managed to finish the installation today, but didn’t have full sun to check max voltage. I did get 180VDC on each string. Just wondering if anyone had specific experiences with a setup where one string would be preferred. At the end of the day it is all about the wattage.

 

 

Which inverter do you have?

 

12x Canadian Solar 275W panels' Voc = 458.4V. 

 

A single string would start producing solar energy earlier in the morning, and later in the afternoons, as apposed to 2 strings, if all other conditions are within "spec", i.e. no shade, PV array facing north, at the desired angle for your location. 

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I have a Goodwe inverter. It can handle the stated Voc.

It makes sense that the incoming voltage will be higher for a longer period during the day. 

I will try it for two days on two strings, and then on one string to see the kWh produced. 

Thank you for the shared knowledge thus far. 

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56 minutes ago, Atti2de said:

I have a Goodwe inverter. It can handle the stated Voc.

It makes sense that the incoming voltage will be higher for a longer period during the day. 

I will try it for two days on two strings, and then on one string to see the kWh produced. 

Thank you for the shared knowledge thus far. 

No, I meant, which inverter model do you have ;) 

The GW5048D-ES and GW3648D-ES has a minimum startup of 150V, whereas the other models can startup at 125V - meaning you can run 2 strings on those, but it would be a bit limited on these the GW5048D-ES and GW3648D-ES earlier in the morning. 

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I am prepared to put some money on the fact that the higher Voltage will win out... Lower wire resistance (lower amps, higher voltage), better choices for the MPPT controller to make (the bigger the range it has much more chance to get the best kW out of panels). From recently acquired knowledge and experience, the closer you can get to the max MPPT range the better. I was running about 180V on a string and was getting pathetic results, and when I jumped to 300+ V on a string I got closer to the panel rated values because the MPPT could work closer to it's best. R10 bucks, payable in reputation points?:D

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KLEVA, that makes complete sense to me. After tomorrow I will change it to 1x12 panel string and validate.

I can see that the solar panels managed to produce 12.4kWh today. Admittedly the generation stopped at 13:00 as the load was low and the batteries fully charged. I then removed the feedback limit at about 15:00. 

Will see how it goes tomorrow. For today, I have produced more than what was used. 

 

Edited by Atti2de
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Wow - even with the 2x6 you winning more than me... I have a nasty shade problem that kills about 3 hours of generation (neighbour refuses to let me get rid of a tree) and generate about 8kW with almost the same panels (2.85kW panels roughly), so quite sure you will see an improvement. Only get about 5 hours a day in Winter and at a bad angle, In Summer hoping to get 8 hours at proper angle and maybe I will get to about 13-15kW (holding thumbs if that damn tree doesn't get to high)... I have a office load that commits me to at least 400W that I cannot get rid of without closing business, and operates 24/7. So not quite off Metro Power yet, but no more roof room and no more budget, so I am at my limits.

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KLEVA, you mentioned something about resistance in the wires earlier. From electronics I recall conductive resistivity being used to calculate the resistance and hence the voltage drop  

At 480VDC and drawing 7A, I calculate a 2.19V (0.46%) over 30 meters. The resistance in the cables stays the same, but now there are two sets and therefore a drop in each string, is 1.09V (still 0.46%), which totals 2.18V. Either way the resistance is the same and therefore the loss the same unless the cables are shortened. 

Make sense or am I thinking nonsense here?

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P = I^2 * R. Power goes up with the square of the current if resistance is constant. Power loss would do the same.

Edit: So basically, half the current is a quarter of the power loss.

Edited by plonkster
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7 minutes ago, plonkster said:

P = I^2 * R. Power goes up with the square of the current if resistance is constant. Power loss would do the same.

Edit: So basically, half the current is a quarter of the power loss.

Understood but I fail to see the correlation with my response. Current will stay at a max of 8.8A in my case. The voltage is more important here as it will allow the inverter to start generation from an earlier time up to a later time as you have more voltage in a 12 panel series string vs. a 6 panel series string. 

If I am missing the boat then please correct me.  Very willing to learn more. 

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2 minutes ago, Atti2de said:

The voltage is more important here as it will allow the inverter to start generation from an earlier time up to a later time as you have more voltage in a 12 panel series string vs. a 6 panel series string.

Oh, absolutely agreed. The reason to have two strings is to make better power when those strings have a different maximum power point, which happens if they don't point quite the same way, has partial shading, and so on. Then you win more by having independent trackers. Otherwise I agree, I'd expect the higher voltage to do better. And it starts earlier too, like you said.

I've just learned not to assume too easily that there might not be a subtle difference in MPP and that counter-intuitively it might just help to have two trackers on it. I don't think it will make more... but I half suspect it will be surprisingly close.

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1 hour ago, Atti2de said:

KLEVA, you mentioned something about resistance in the wires earlier. From electronics I recall conductive resistivity being used to calculate the resistance and hence the voltage drop  

At 480VDC and drawing 7A, I calculate a 2.19V (0.46%) over 30 meters. The resistance in the cables stays the same, but now there are two sets and therefore a drop in each string, is 1.09V (still 0.46%), which totals 2.18V. Either way the resistance is the same and therefore the loss the same unless the cables are shortened. 

Make sense or am I thinking nonsense here?

Are you really running 30m of cable?

And, with 2 strings, you now have two sets of 30m cables, which effectively gives you double ampere capacity, though you might not need it. 

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I've thought of another reason why two strings might give you more. One assumption about the higher voltage string is that all the modules are identical. Are they? If one of them is off by 5% it derates the entire array by 5%. It might well be that separate trackers gives you a tiny bit more then. But in the simplest theoretical case, the high voltage option should give you more.

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6 hours ago, SilverNodashi said:

Are you really running 30m of cable?

And, with 2 strings, you now have two sets of 30m cables, which effectively gives you double ampere capacity, though you might not need it. 

There are two strings and each one has a set of cables from a set of 6 panels. If I had to remove one string and series the panels on the roof then I will have 15m less cables (and therefore less losses). 

I understand your point - it does make sense. Let us see how big an influence the 0.46% loss has. 

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3 hours ago, Atti2de said:

There are two strings and each one has a set of cables from a set of 6 panels. If I had to remove one string and series the panels on the roof then I will have 15m less cables (and therefore less losses). 

I understand your point - it does make sense. Let us see how big an influence the 0.46% loss has. 

I was just curious whether you are indeed running 30m of cabling? But I guess it's 30m in total then? In which case you should do the math based on the 15m, or actual cable run from the panels to the inverter. 

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OK!

Some results... these show the kWh produced for the days by the PV's.

Day 1: 12.7kWh (2x6)

Day 2: 14.5kWh (2x6) 

Day 3: 15.4kWh (1x12)

It seems as if the higher voltage wins. I think the simple reason for this is not because of resistance in wires or anything, but simply because the inverters' MPPT starts generating power above a certain voltage.

image.thumb.png.8e3e1b19937e9643f0b1e8320357320f.png

image.thumb.png.60d6e930e79985fcaeb8652fe75772bd.png

chart.thumb.jpeg.c13b21749c091807320dc7c16f0a3708.jpeg

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1 hour ago, Atti2de said:

OK!

Some results... these show the kWh produced for the days by the PV's.

Day 1: 12.7kWh (2x6)

Day 2: 14.5kWh (2x6) 

Day 3: 15.4kWh (1x12)

It seems as if the higher voltage wins. I think the simple reason for this is not because of resistance in wires or anything, but simply because the inverters' MPPT starts generating power above a certain voltage.

 

image.thumb.png.60d6e930e79985fcaeb8652fe75772bd.png

 

I see your 6:30am load for your kettle is the same as mine ;-)

Just over 2kW

 

image.thumb.png.12ea5d453e62aed3e96834ef1f607a90.png

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On 2018/08/22 at 8:45 PM, Antony said:

I see your 6:30am load for your kettle is the same as mine ;-)

Just over 2kW

Try living in a household with 2 non-functioningworking adult children (26 & 28), who just sit around and steal your coffee all day. Forget they turned the kettle on 10 min ago, so it must go on again!!! Counted 38 times one day before the Wife lost it! She has started locking the kettle away in the FRIDGE (we lock our fridge due to the same "ahem" adults) before she goes to work.

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