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Am considering installing 3 kVA pv system on my roof top. I am to use a string system on a single inverter.

My choice is : 12 by 250 W =3000 W or 10 by 300 W =3000 W

Space is not an issue. installer told me 12 are better as overall efficiency would be better.

I am not sure why. I am quite technical but cannot reason this.

Tks

Fred
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NotForFun
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    In the absence of any other differences between the panels, you're right that it doesn't make sense that 12 are more efficient than 10. But are there any other differences? For example, are both panels the same manufacturer? The same type of panel (polycrystalline, CdTe, monocrystalline, amorphous silicon)? – 410 gone Jul 01 '15 at 18:45
  • Tks for your sound comments. We are comparing same brand but different models. Both Polycrystalline and efficiency 15.7%. – NotForFun Jul 02 '15 at 06:06
  • Puzzling: Here are some factors which can matter but do not appear to here: > 250W panels are usually 30V MPP and 300W are usually 36V Voc. So 12 x 30V = 360V and 10 x 36V = 360V so essentially the same. > Some inverters have a MPPT range which panels may fall outside but should not matter here. > Higher V = less I = less I^2 losses but ... . IF area larger per Watt then temp may be lower which give more power but seems not so here. If anything - 12 will use more connectors and more cable so SLIGHTLY higher resistance so I^r losses higher. So - seems strange. ... – Russell McMahon Jul 23 '15 at 14:05
  • ... 12 does give you slightly better redundancy long term if one panel damaged or goes low output due to manufacturing problem etc. 12 has slightly less shading impact for same shadow area as less % of cells are diode bypassed (*probably). – Russell McMahon Jul 23 '15 at 14:05

2 Answers2

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Summary: like you, I'm sceptical, but it is plausible.

In the absence of any other differences between the panels, you're right that it doesn't make sense that an array of twelve 250W panels is more efficient than an array of ten 300W panels. Given that it's the same brand of panel, the same efficiency of panel, and the same type of panel - polycrystalline - it's very hard to see why 12 panels would be more efficient.

It's conceivable that your supplier can just get the 250W panels cheaper, and so is trying to nudge you in that direction. Or, if some part of the array might be subject to shading during part of the day, perhaps the array of twelve panels might just generate more power when there is partial shading.

Or it could be that there's the inverter operates with different efficiency depending on the input voltage; the input voltage is driven by the number of panels you have in series (assuming the panels have the same voltage, and the 300W panel just gives a higher current). Twelve panels gives the option of installing 1,2,3, 4, 6 or 12 in series; whereas an array of 10 panels could be 1, 2, 5 or 10 in series. The inverter might operate more efficiently with one of the formations unique to a 12-panel array (3, 6 or 12 in series) than any of the formations available to a 10-panel array. This would not apply in the case of say, the Suntech panels, where the 300W panel seems (according to my reading of the datasheet) to scale up voltage rather than current, so 12 250W Suntech panels in series have the same voltage as 10 300W Suntech panels.

410 gone
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  • Could be that the supplier may have a better deal with one model or another. Re shading, the less panels you use the less probability of shading so its in favour of the 10 panels. As they are connected in series with 1 inverter, a partial shading on 1 panel would influence the whole system. Ideally split with multiple inverters but cost would be more and I bet also overall efficiency would be compromised slightly too. – NotForFun Jul 02 '15 at 06:36
  • Ah, is the proposal that either the ten or the twelve would all be in series? – 410 gone Jul 02 '15 at 06:41
  • The datasheets for panels are: – NotForFun Jul 02 '15 at 07:24
  • The datasheets for panels are: link and (250w) link . Inverter is: link . A series configuration is to be used. Inverter max input voltage is 500v. 300w panel open circuit voltage is 45.3 (sum 450v) and 250w 38v (sum 456). So within range and close. – NotForFun Jul 02 '15 at 07:36
  • Then have another quote for same price claiming more units but warranty period is less. http://s-energy.designq.kr/download/SE-Datasheet_SN310P-10_3BB_72_2015_EN_V01.1.pdf using set of 10 S-energy panels of 310w each. – NotForFun Jul 02 '15 at 07:40
  • Actually partial shading would favour the 10 panel setup. – Aron Jul 02 '15 at 16:59
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Actually I have bought 10 panels of S-Energy at 310W each. http://s-energy.designq.kr/download/SE-Datasheet_SN310P-10_3BB_72_2015_EN_V01.1.pdf

One supplier told me that the 250w would be better than 310 size divided/area you get more Watts/m2.

Considering that space has also a cost, I preferred using 10 panels rather than 12. If in the next coming years technology and price improve I would have enough space to add 2 more panels maybe using micro inverters.

NotForFun
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