Hey all,

 

So I need a bit of assistance, I am not sure how to calculate the space I would need to leave between panels on a flat roof to prevent shadowing - especially in winter when the sun is lower.

I have googled and found a calculator, but it is for people in the Northern hemisphere. Does anyone have a calculator which would work for us here in SA?

I would expect that the distance would be the same whether in the northern or southern hemisphere, the max sun angle would be the same for the same latitude N or S.

1 minute ago, jumper said:

I would expect that the distance would be the same whether in the northern or southern hemisphere, the max sun angle would be the same for the same latitude N or S.

I really do not know, I dropped Geography in grade 9. lol

 

However when using the online calculator vs trying to follow their formula, I ended up with different figures - not sure if it is North vs South or if I could not follow the formula properly. Also isn't the Earth tilted slightly which could affect the angle of the sun?

13 minutes ago, jumper said:

I would expect that the distance would be the same whether in the northern or southern hemisphere, the max sun angle would be the same for the same latitude N or S.

Partly true if you are the same degrees away from the equator. 

@PsyCLown

In order to do the sums you need to provide the size in mm of the long dimension of panels and also the angle that you want to lift the panels on a frame. Short side if fitting landscape. It will help to provide the dimension of the roof too. 

If you did not drop maths it should be easy to work out 😀😀😀

Edited July 10, 20233 yr by Scorp007

2 minutes ago, PsyCLown said:

Also isn't the Earth tilted slightly which could affect the angle of the sun?

Yes, this tilt is what gives us the 4 seasons, it's just 6 months difference between N and S, but the lowest angle of the sun is the same for everyone on the same latitude N & S, just 6 months apart. In June the Northern hemisphere is tilted towards the sun and they have the highest sun angle and we have the lowest in our winter. In December the Northern hemisphere is tilted away from the sun so they have their lowest sun angle (Winter solstice) and we have our highest in the middle of our summer (Summer solstice).

Angles are the same at the same latitude, just 6 months shift due to the tilt.

On 2023/07/10 at 3:29 PM, Scorp007 said:

Partly true if you are the same degrees away from the equator. 

@PsyCLown

In order to do the sums you need to provide the size in mm of the long dimension of panels and also the angle that you want to lift the panels on a frame. Short side if fitting landscape. It will help to provide the dimension of the roof too. 

If you did not drop maths it should be easy to work out 😀😀😀

I was thinking of placing the panels in portrait, the height of the panels are 2465mm. Tilt angle on the frame is 25 degrees.

Width is 1134mm, not sure if I should rather consider mounting them in landscape - if this way I will be able to mount more without shading?

 

The roof, well, I used Google maps for some measurements.

 

 

 

 

The blue is a wall / parapet on the roof, which could throw a bit of shade. The red is my geyser so not usable space.

The tree has been cut back a bit and will continue to be cut back to help prevent shading when I get around to adding panels in that section (probably populate that section last).

The small triangle is the roof for my patio, I reckon one or 2 panels at max in that space.

 

I have highlighted my roof in yellow, as tempting as it is to use the roof of my neighbour behind me - I do not think they will appreciate that when they find out. lol

 

Here is a picture to put things into perspective, taken from me standing on that triangle section of the roof. Picture was taken around 7.30AM so sun was still rising.

 

 

Edited July 13, 20233 yr by PsyCLown

18 hours ago, PsyCLown said:

I was thinking of placing the panels in portrait, the height of the panels are 2465mm. Tilt angle on the frame is 25 degrees.

sin(25)x2465 = 1042mm : Height at the back of the panel in portrait

18 hours ago, PsyCLown said:

Width is 1134mm, not sure if I should rather consider mounting them in landscape - if this way I will be able to mount more without shading?

sin(25)x1134 = 480mm : Height at the back of the panel in landscape.

(Someone please double check my math, it's been a very long time since I was in school 😉 )

The gap between the panels will be smaller in landscape, but you will be able to fit fewer next to each other.

You would now need to use the calculated height of the panel along with your latitude to get the distance between the panels in portrait and landscape and plan further from there.

It might be a good idea to either lift the panels a bit to reduce the shading created by the wall around the roof or you'll need to leave a space. It might be best to lift them and put them in landscape so they don't stick up too high. Any lift would need to be added to the original height in the formula as the spacing will increase. You also need to add the height of any footings for the frame.

You might also need to plug the height of the northern parapet wall (the one your're standing on in the pic) into the calculator to see the space you need to leave for the first panel so that shadow doesn't get it in Winter. It might be best to get those boundaries first and then see what space you have left to work with.

Edited July 11, 20233 yr by jumper

Have a chat with @Steve87 if you are in JHB area.

 

3 hours ago, PsyCLown said:

Here is a picture to put things into perspective, taken from me standing on that triangle section of the roof. Picture was taken around 7.30AM so sun was still rising.

That roof top looks very reflective. Maybe you should consider bi-facial panels. 

4 hours ago, PsyCLown said:

I was thinking of placing the panels in portrait, the height of the panels are 2465mm. Tilt angle on the frame is 25 degrees.

Width is 1134mm, not sure if I should rather consider mounting them in landscape - if this way I will be able to mount more without shading?

 

The roof, well, I used Google maps for some measurements.

 

 

The blue is a wall / parapet on the roof, which could throw a bit of shade. The red is my geyser so not usable space.

The tree has been cut back a bit and will continue to be cut back to help prevent shading when I get around to adding panels in that section (probably populate that section last).

The small triangle is the roof for my patio, I reckon one or 2 panels at max in that space.

 

I have highlighted my roof in yellow, as tempting as it is to use the roof of my neighbour behind me - I do not think they will appreciate that when they find out. lol

 

Here is a picture to put things into perspective, taken from me standing on that triangle section of the roof. Picture was taken around 7.30AM so sun was still rising.

 

This is a good starting point. Some installers have software that they use to give the answers. You also need to provide coordinates of location even it is 1 degree away as to not give your near location. 

Does the top of the picture face North? 

@jumper

Yes your 2 calcs at 25 deg angle is correct. Now one has to work with the sun at about 36 degrees from horizontal to get the length of the shadow from the top of the panel. 

Edited July 10, 20233 yr by Scorp007

9 minutes ago, Scorp007 said:

Yes your 2 calcs at 25 deg angle is correct. Now one has to work with the sun at about 36 degrees from horizontal to get the length of the shadow from the top of the panel.

Thanks for the double check 👍 Yes, ideally we need the exact latitude to get the lowest sun angle and then calculate the length of the shadow.

11 minutes ago, Scorp007 said:

Does the top of the picture face North?

The picture faces kinda SW, sun rising over the left shoulder behind him. Basically the panels will face the camera.

good rule of thumb for our latitude: skip distance to be 1.8 x the panel difference in height...

this ensures shade free panels from alternate rows from 9am til 3pm in the worst winter tilted months.

for sunrise +1 hour and sunset -1 hour you need to multiply by 3.2.

A hep of lot of ppl get bogged down with Trigonometry, get real, we need tangible rule of thumbs. 

Edited July 10, 20233 yr by Steve87

This is the formula I was taught to use. 

Hope it helps.

Thanks guys, so basically what @Steve87 and @Scabanshi have posted work on the same concept, although with different factors.

Steve having a factor of 2.3 and this should ensure no shading from 9am to 3pm in winter. Scabanshi a factor of 1.7 which will likely provide no shade for fewer hours.

 

So if I work on the approx height of 1042mm as calculated earlier, with Steve's formula it leads to 2.4m space between panels and Scabanshi 1.8m between panels.

Now I need to work out whether I should go for portrait or landscape mounting of the panels to maximize space and PV  generation.

 

My latitude is 26.00 degrees basically

 

 

@PowerUser I was considering bi-facial panels, however I had some questions regarding a bi-facial panel. I assume there is still only 2 MC4 connectors per panel (positive, negative), not sure how voltage would differ between just the front of the panel getting sun vs some on the rear and how to calculator the maximum voltage to ensure one stays within the safe limits of the MPPT?

13 minutes ago, PsyCLown said:

I was considering bi-facial panels, however I had some questions regarding a bi-facial panel. I assume there is still only 2 MC4 connectors per panel (positive, negative), not sure how voltage would differ between just the front of the panel getting sun vs some on the rear and how to calculator the maximum voltage to ensure one stays within the safe limits of the MPPT?

I don't have experience with bi-facial panels but when I saw your shiny white roof, it looked ideal for some of those. I assume the calculations are the same as for any other panel. You just have to check the correct voltage and current for them from the datasheets.

So just to chuck a pebble in the pond. Just lying the panels flat might be more economical than having them on expensive racks. If the panels cost a couple of thousand and the panels are roughly the same cost, it might be cheaper or similar cost just to lie the panels flat. There is more space for panels and the money for the racks go toward more panels.

Edited July 11, 20233 yr by HendrikBigChief

16 minutes ago, PowerUser said:

I don't have experience with bi-facial panels but when I saw your shiny white roof, it looked ideal for some of those. I assume the calculations are the same as for any other panel. You just have to check the correct voltage and current for them from the datasheets.

I looked into it again and with my Luxpower inverter, I am not sure it would be worth it as the MPPT is 13A max and the bifacial panels I can find, the front of the panel maxes out at 13A and depending on the rear panel production, goes upto 17A. So a lot of the potential power could be clipped. Seems the voltage remains consistent but the current increases, so it would work better with a larger inverter which typically has higher current limits on the MPPT.

 

For the price of those panels, I can get larger monofacial panels ie. 620W monofacial vs 550W bifacial.

 

 

@HendrikBigChief I wonder what the production would be like with a panel flat on the roof, however the mounting system is not terribly expensive compared to the panels also I would run out of capacity on my MPPT so that would be a limiting factor as well.

Mounting system for 10 panels in portrait works out to approx R13k for the mounting structure. So I would be able to add an extra 3 panels with some change by doing this.

3 minutes ago, PsyCLown said:

I looked into it again and with my Luxpower inverter, I am not sure it would be worth it as the MPPT is 13A max and the bifacial panels I can find, the front of the panel maxes out at 13A and depending on the rear panel production, goes upto 17A. So a lot of the potential power could be clipped. Seems the voltage remains consistent but the current increases, so it would work better with a larger inverter which typically has higher current limits on the MPPT.

Yes, maybe they are not ideal for your inverter. You have to match the panels to the inverter requirements.

 

1 hour ago, PsyCLown said:

Thanks guys, so basically what @Steve87 and @Scabanshi have posted work on the same concept, although with different factors.

Steve having a factor of 2.3 and this should ensure no shading from 9am to 3pm in winter. Scabanshi a factor of 1.7 which will likely provide no shade for fewer hours.

 

So if I work on the approx height of 1042mm as calculated earlier, with Steve's formula it leads to 2.4m space between panels and Scabanshi 1.8m between panels.

Now I need to work out whether I should go for portrait or landscape mounting of the panels to maximize space and PV  generation.

 

My latitude is 26.00 degrees basically

 

 

@PowerUser I was considering bi-facial panels, however I had some questions regarding a bi-facial panel. I assume there is still only 2 MC4 connectors per panel (positive, negative), not sure how voltage would differ between just the front of the panel getting sun vs some on the rear and how to calculator the maximum voltage to ensure one stays within the safe limits of the MPPT?

The calc from @Scabanshi

will also work fine and takes less space. My spacing is much less and for N facing panels at 28deg only has a shadow for about 30min for around 5 weeks in winter. 

Sometimes it is just practical not to break your neck to go for the ideal distance just as cutting down a whole tree vs just trimming it from time to time. 

2 great simple methods from both members

Just my 2c.

Edited July 11, 20233 yr by Scorp007

17 hours ago, jumper said:

Any lift would need to be added to the original height in the formula as the spacing will increase. You also need to add the height of any footings for the frame.

@PsyCLown, obviously you don't need to add any lift to the calc if you are lifting all the panels, what was I thinking 🤦‍♂️

I would go with 1.7x as posted by @Scabanshi as 22deg is a bit further North than JHB. (sorry I misread here, you're at 26deg which is the same as Jhb) Do the calcs with 1.7x and you'll probably end up with extra space an the end and then you can space them a little more.

You can also use 1.7x(height of north wall-panel lift) to get the placement of the first row out of its shadow.

Thanks @Steve87and @Scabanshi for these easy rules, I was taking the long way round 😀

Edited July 11, 20233 yr by jumper

@PsyCLown

Now you can work out all your panel positioning. 

My rough estimate is 3 rows with 4+4+2 and then 5 along the portion where you have the 12.9m dimension line for a total of 15 in portrait. 

Edited July 12, 20233 yr by Scorp007

Just a thought - you will need to maintain/paint your flat roof waterproofing layer every 3 years if you have an existing warranty on the waterproofing..

Take that into account when considering the layout.

3 hours ago, FixAMess said:

Just a thought - you will need to maintain/paint your flat roof waterproofing layer every 3 years if you have an existing warranty on the waterproofing..

Take that into account when considering the layout.

I was thinking one could simply paint over the frame brackets and not necessarily need to move the brackets. As under the brackets where the paint could not be applied, would sort of be protected from the sun by the panels and the aluminium.

Although I would need to ensure that there is easy access behind the panels to get in that area to paint the roof.

16 hours ago, Scorp007 said:

@PsyCLown

Now you can work out all your panel positioning. 

My rough estimate is 3 rows with 4+4+2 and then 5 along the portion where you have the 12.9m dimension line for a total of 15 in portrait. 

I was thinking maybe do portait on the outskirts of the wall and then possibly landscape everywhere else to try and fit more panels - will need to do some maths and work out what will allow me to fit the maximum number of panels. I'd like to at least aim for 20 panels if possible, if I use 620W panels that is 12.4KW of panels.

 

I'd love even more space for more panels, but roof space is unfortunately limited.