Panel Confusion

[ianlloyd100]

So.. I'm looking to put solar on the roof of the Motorhome. I've found 2 different 300+ watt panels, but the specs are very different... Which is better?. Not price wise, but technically, and why?..

12V solar panels charging kits for caravans, motorhomes, boats, yachts, marine

12V solar panel solar charging kits for motorhome caravan boat campervan yacht marine off-grid

www.photonicuniverse.com

Solar panel specifications:
• Peak power: 320W
• Maximum power voltage: 18.35V
• Maximum power current: 17.88A
• Open circuit voltage: 21.85V
• Short circuit current: 19.04A
• Power allowance range: +/- 3%
• Dimensions: 179 x 99 x 4 cm
• Weight: 19.5 kg
• 8 mounting holes 9 x 14 mm
• 2 x 5m of high quality single core solar cable (6.0mm cross section)
• Male and female MC4 connectors

Midsummer Energy

midsummerenergy.co.uk

Specifications:
• Peak power Wp 320W
• Voltage at peak power Vmp: 34.01V
• Max open circuit voltage Voc: 41.55V
• Current at peak power Imp: 9.41A
• Max short circuit current Isc: 9.77A
• Weight: 18kg
• Dimensions: 1640 x 992 x 35mm
• Warranty on output: 25 years
• Product warranty: 12 years

Looking for some sensible help.........

Thanks..

Ian.

 

[Craig]

My default would be to go for the higher volts for 2 reasons.
1. You have the choice to go 24 volt system with using 1 panel this will help with possible shading issues.
2. Higher volts means less amps and smaller cable sizes required.

 

[ianlloyd100]

Craig.
Thanks for the very swift reply. I was hoping you would say that... The package deal looks very good value for money....

 

[Dzl]

My default would be to go for the higher volts for 2 reasons.
1. You have the choice to go 24 volt system with using 1 panel this will help with possible shading issues.

Can you explain how going with a 24 volt system helps with shading?

 

[Craig]

The 24 volt doesn't help with shading other than that if you get higher voltage panels you can run them in parallel instead of in series to make 24Volts. Because of this if one panel is under agree a the other will still produce. If they are in series and one is shaded you will get almost nothing from second panel that is not in shade.

 

[April]

The 24 volt doesn't help with shading other than that if you get higher voltage panels you can run them in parallel instead of in series to make 24Volts. Because of this if one panel is under agree a the other will still produce. If they are in series and one is shaded you will get almost nothing from second panel that is not in shade.

Thats for old panels . Newer panels with bypass diodes will allow the series panel to output its full capacity despite the other in shade so you need to qualify that statement a bit now.

 

[Dzl]

Thats for old panels . Newer panels with bypass diodes will allow the series panel to output its full capacity despite the other in shade so you need to qualify that statement a bit now.

Is there any way to tell whether the panels have bypass diodes or not from looking at the panels or from the specs sticker on the back of the panel? The panel I'm looking at was produced roughly between 2013-2017 by Sunpower. Not sure if this qualifies as a 'newer panel.'

edit: never mind, I was able to find an answer to my question. Each panel apparently has 3 bypass diodes, seems to be true for many other models of Sunpower panel as well.

This brings up another question, what is the significance of the number of bypass diodes on a panel?

 

[gnubie]

Once the shading gets bad enough the current from the other panels in the string (or just that one panel if that's all you have) will flow through the diode instead of the shaded cells. Shaded cells act like resistors in that they reduce current flow. The more diodes the more zones the panel has that can be bypassed so you get more production from the panel.

If there was just one bypass diode across the entire panel once the shading got severe enough to permit current to flow through the diode you would essentially lose all the production of any cells in the panel that were still well lit.

 

[Dzl]

Once the shading gets bad enough the current from the other panels in the string (or just that one panel if that's all you have) will flow through the diode instead of the shaded cells. Shaded cells act like resistors in that they reduce current flow. The more diodes the more zones the panel has that can be bypassed so you get more production from the panel.

If there was just one bypass diode across the entire panel once the shading got severe enough to permit current to flow through the diode you would essentially lose all the production of any cells in the panel that were still well lit.

Thanks Gnubie, this is a super easy to understand answer. I appreciate the clarity. If I understand correctly 1 diode would create 2 'zones,' 3 diodes would equal 4 'zones' etc.

 

[gnubie]

Really depends on how the diodes are set up. I had an old panel here that had 3 diodes, but one diode bypassed the entire panel so 3 zones for it.

 

[Craig]

Thats for old panels . Newer panels with bypass diodes will allow the series panel to output its full capacity despite the other in shade so you need to qualify that statement a bit now.

Yes but if the whole panel is covered there is no way 2 panels in series will achieve 2x the volts. I understand about the diodes. Some of us live where trees and shading are a real issue not just a theoretical one.

 

[Dzl]

Yes but if the whole panel is covered there is no way 2 panels in series will achieve 2x the volts. I understand about the diodes. Some of us live where trees and shading are a real issue not just a theoretical one.

Seems like for mobile applications (my situation), where shade is unpredictable and unavoidable at times, planning a system that minimizes the performance penalty of partial shade would be important..

Sounds like higher voltage panels (relative to batter bank voltage), wiring in parallel where possible, and purchasing panels with higher numbers of bypass diodes (or something like the Sunpower P-series) are all smart choices. Anything I'm overlooking?

 

[ianlloyd100]

Thanks for the answers, and some extra knowledge on the Shading stuff... Don't think it's going to be a problem for me, but interesting none the less....
I ordered the Higher Voltage panels which looks like a good value for money package. I'll let you know how I get on ...