Connecting solar panels on different sides of roof

vlovic

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May 7, 2021
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Hello,

I want to add solar panels to my roof but to get enough power I need to use different sides of the roof such that some panels will be shaded while others will not. Connecting the panels in series will reduce the efficiency then. I could add solar panel optimisers, but these are quite pricey and I am looking for a low cost solution.

Can I connect groups of panels from the same side of the roof in series, and then connect the different groups (from each side of the roof) in parallel? Do the different groups all need to have the same number of panels to keep the voltage constant? Or can I have e.g. 3 panels in parallel with a group of 5 panels?

I'd be grateful for any advice, thanks in advance.

Victor
 

wattmatters

Photon Sorcerer
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Apr 16, 2021
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Can I connect groups of panels from the same side of the roof in series, and then connect the different groups (from each side of the roof) in parallel?
Yes, provided they all are within the voltage and current specifications for the inverter's MPPT inputs.

It works best if the orientation of arrays is not too far apart. e.g. One string east and one string west each on a 60 degree pitch roof isn't great but on a regular lower pitch roof it's fine.

Do the different groups all need to have the same number of panels to keep the voltage constant? Or can I have e.g. 3 panels in parallel with a group of 5 panels?
Keep voltage constant between strings.

Good practice:
- All panels in a string should be of the same rating, orientation and tilt.
- Strings which are connected in parallel should each have the same number of panels.

If you have significantly different sized arrays then use two MPPTs to separate the strings.
 

chrisski

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In my signature block there’s a link with math to mismatched panels.

Voltage needs to be the same on each string or you could lose a lot of power. Basically for the math, the lower voltage wins.

If you put three panels on parallel with five on the other, this sound like a huge voltage mismatch and should go four and four.

please note in some cases two 100 watt panels in parallel with a single 200 watt panel is fine provided voltages are close. A lot of the mismatched panels I look at, the voltages are not close.
 

chrisski

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What I don’t remember from the video is how to size a blocking diode and which way the striped end goes.
 

chrisski

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It depends on the indidual panel or string size if installed in a combiner box. Look here for guidance
That was similar to Will’s video. Would be nice to see a video with very specific placement and sizing.

For example, she mentioned diode for voltage and amperage, but which one? Short circuit voltage or operating voltage and if the exact voltage is not available go up or down In size? Is it the same for amperage? There was enough in the video to figure which way to place the diode. She mentioned there is a loss using one, Will was a little more specific about a couple watts which is negligible.
 

Austint

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Aug 1, 2021
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Suppose two similar arrays are on two sides of the roof and connected together with blocking diodes into a mppt input . As the sun progresses across the sky will the power available into the inverter be the same as using 2 mppt inputs ?
 

brewmatic

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Aug 6, 2021
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I'd connect panels on one side in series and use a separate charge controller for each side. This way you get efficiency close to power optimizes.
 

chrisski

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Shows here at 1:18 the ring side is how he describes it
What's missing from this is how he chose the N4001 blocking diode. I'm sure 300 watts of panels needs a different diode than 1.5 watts like his video. I'm also sure the 300 watt higher voltage lower amp panel's diode is different than the 300 watt lower voltage higher amperage panel's diode.

I'm not sure of the exact VOC or VMP of my panels, but operate around 50 volts and up to six amps.
I'd connect panels on one side in series and use a separate charge controller for each side. This way you get efficiency close to power optimizes.
What I have now is three charge controllers. 1 is maxed out, the other two have some room for expansion. These RVs are a Tetris puzzle getting panels up there. I'd rather figure the blocking diode thing out to add a couple more series of panels than get a fourth and fifth charge controller.

For me it would be getting enough panels to run the AC without getting a fourth and fifth SCC.

I can also see a use for parallel panels and blocking diodes for huge systems to max those out.
 

wattmatters

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The roof can't be tightly packed with panels, as on hot days the heat generated can leak through your attic and cause it to overheat.
Solar panels provide significant shading to the roof and reduce heat transfer into the ceiling by quite a lot. Also a nice chunk of the radiant energy hitting the panel is converted to electricity.

Correctly mounted on rails, there will be plenty of air circulation under panels. Usually the panel is raised a minimum of 10cm above the roof for fire rating reasons. Standard racking systems will do this without problem.

Moreover, there should be a gap of 12 inches or one foot between the last row of solar panels and the edge of the roof to ensure that the panels can fit as they expand and contract during the day.
Panel's proximity to roof edges, gutters and ridge lines should have an exclusion zone no smaller than the local codes require but it has nothing to do with panels expanding. It's about wind loading and other structural and safety reasons.

And for the same reason, there should be around 4-7 inches of space between two panels.
You might be thinking about the gap between rows of panels, not between two panels.

Most panel specs require a minimum gap of ~10mm (0.4") between panels. That's all that's required to allow for thermal expansion. Typical gaps with standard roof racking systems provide 15-30mm between panels.
 
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