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diy solar

Panel Angle: Northern Hemisphere

EclecticBadger

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Joined
Sep 20, 2019
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I have always used the following advice as means of judging the most efficient angle to the horizontal for northern hemisphere and my UK location (latitude 53 degrees):

SUMMER
Degrees Latitude x 0.9 - 23.5

SPRING / AUTUMN
Deg. Lat. - 2.5

WINTER
Deg. Lat. x 0.9 + 29.0

Simply put, summer more horizontal, winter more vertical.

Generally fixed panels should face as near due South as possible. However, for systems capable of solar tracking and re-orientating compass direction:

MARCH / SEPTEMBER
Sunrise due East. Sunset due West

Late-JUNE
Sunrise North-East. Sunset North-West

Late-DECEMBER
Sunrise South-East. Sunset South-West

My recommended Best App for locating where the sun should be on overcast days for Android and iOS: Sun Surveyor by Adam Ratana.
 
I like SAM (NREL's free System Advisor Model), with it you can pretty easily get the optimum tilt and azimuth (160 degrees here instead of 180).

It also uses historical weather data, for example there's no sense factoring in July's extreme angle if it rains every afternoon in July.

But, what I've found in my hurricane prone area is that even the slightest bit of upwards tilt doubles the down-force on the roof.

So, in the end I'm going for 0 tilt, having to periodically clean my panels (heard you need > 10% for self-cleaning), and losing about 10% power. A couple of extra panels to make up the difference vs a heard of elephants on the roof.

Here's some sample numbers with 185 mph winds (units are pounds force per attachment point, e.g., 20 tons at 5% with 64 attachment points):

tilt row space down up lateral momentum
0 0 201 539
5 6 648 508 56 43
10 8 727 607 128 75
15 10 778 727 208 110
20 12 862 848 313 157
0*OiYwLzOLeBKucylR.jpg

It's time to adjust the Tilt!​
 
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I have similar weather problems so panels are parallel with the roof with wind rated racking. I recently climbed up to give all the nuts a rattle to make sure they were still tight and the panels were pretty dusty, but a bit of rain if it ever comes would take care of that. The biggest problem was birds had crapped all over a few of them, or one bird blew up midflight.
 
This [SAM] really is the all you can eat nuke to crack a nut and seems just the job for roof mounted grid tie-in systems and large arrays.

Fortunately, the need to calculate roof stress due to tornado down force is not something commonly experienced in the UK, although we do have some pretty forceful storms and I wondered if any of those weather data sets extended beyond the US?
 
Not so nuclear powered I guess. I got the roof loading numbers from the IronRidge racking website. They have a couple of "designers" there. AFAIK, SAM doesn't do wind loading (more's the pity, that is amazingly hard to figure out). With IronRidge you enter the windspeed and it gives your the force per attachment point. From there you just multiply by the number of attachments.

The Weather SAM uses does take rain and such into consideration though.

There's a link inside the program for weather sources outside the US: https://sam.nrel.gov/weather-data, says it can use European sources. Let us know if you find something!
 
Where abouts are you located?
I have always used the following advice as means of judging the most efficient angle to the horizontal for northern hemisphere and my UK location (latitude 53 degrees):

SUMMER
Degrees Latitude x 0.9 - 23.5

SPRING / AUTUMN
Deg. Lat. - 2.5

WINTER
Deg. Lat. x 0.9 + 29.0

Simply put, summer more horizontal, winter more vertical.

Generally fixed panels should face as near due South as possible. However, for systems capable of solar tracking and re-orientating compass direction:

MARCH / SEPTEMBER
Sunrise due East. Sunset due West

Late-JUNE
Sunrise North-East. Sunset North-West

Late-DECEMBER
Sunrise South-East. Sunset South-West

My recommended Best App for locating where the sun should be on overcast days for Android and iOS: Sun Surveyor by Adam Ratana.
 
Great info buddy. Thanks for that.

I currently have 6 LGs on the roof of my truck which in the good weather is not a problem to achieve a good reserve in the batteries but these quickly shortening evenings are killing me - especially if I have been a hermit that day.. I do have a mahoosive fridge freezer though which I should consider changing and no doubt does eat alot.
 
But, what I've found in my hurricane prone area is that even the slightest bit of upwards tilt doubles the down-force on the roof.

Damn! I hadn't thought the wind would make a difference to the effective weight of my roof mounted panels. My roof is steel sheets on wooden purlins, sloping about 15% (a pretty standard steel framed barn structure). 12 * 18.5Kg in panels shouldn't be a problem, but how do I calculate what extra downforce arises from wind? Do I need to calculate it at all, considering the panels are parallel to the roof slope?
 
... how do I calculate what extra downforce arises from wind? Do I need to calculate it at all, considering the panels are parallel to the roof slope?
It's always good to know what the down/up forces are on your panels and probably required for any county inspection.

I'm certainly not an expert but this will help you out: Basics of Wind Loading, I think these are the calculations you'll need to show to get the county to approve your building permits.

Although, before I came across that I cheated. I went to the ironforge racking site and used their designer to calculate the racking I'd need for local conditions at various angles. It tells you the force on each bolt and how many bolts you need. So, that can also give you total forces.
 
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Although, before I came across that I cheated. I went to the ironforge racking site and used their designer to calculate the racking I'd need for local conditions at various angles. It tells you the force on each bolt and how many bolts you need. So, that can also give you total forces.

Great idea! Thanks. I'll take a look there.
 
I have always used the following advice as means of judging the most efficient angle to the horizontal for northern hemisphere and my UK location (latitude 53 degrees):

SUMMER
Degrees Latitude x 0.9 - 23.5

SPRING / AUTUMN
Deg. Lat. - 2.5

WINTER
Deg. Lat. x 0.9 + 29.0

Simply put, summer more horizontal, winter more vertical.

Generally fixed panels should face as near due South as possible. However, for systems capable of solar tracking and re-orientating compass direction:

MARCH / SEPTEMBER
Sunrise due East. Sunset due West

Late-JUNE
Sunrise North-East. Sunset North-West

Late-DECEMBER
Sunrise South-East. Sunset South-West

My recommended Best App for locating where the sun should be on overcast days for Android and iOS: Sun Surveyor by Adam Ratana.

When you come up with the number from your formula, where is that measured from vertical or horizontal, which one is zero?
Crude drawing?

Thanks
 
When you come up with the number from your formula, where is that measured from vertical or horizontal, which one is zero?
Crude drawing?

Thanks
Horizontal to the ground. It is good that you are looking into tilting.

What happens when you don't tilt is you reduce insolation. The cos(angle) will tell you how much you might get if you don't tilt; here is a quick table:

Don't tilt angle / % you get
15 / 96
30 / 86
45 / 70
60 / 50

I am at a 60degree location and that 50% rule holds true enough. Only on the best solar days do I reach 50%. Now, with repetitious cold fronts I don't expect many 50% days.
 
  • What is ideal for solar panel energy generation?
    • A panel that is always at a right angle to the sun.
  • Do you have to adjust every day to get it perfect?
    • Yes
  • Is it worth your effort to adjust it every day to get it perfect?
    • It depends.
      • If you have 45 megawatts, yes.
      • If you have 4500 watts, no.
  • A fine compromise with no adjusting is your latitude (The angle of the September/March Equinox)
    • You get a summer bias.
  • If you want a 2 angle system a little better compromise
    • adjust in October and April
      • Adjust your panels 2 times a year, your latitude + or - 8°.
  • If you want a 3 angle great compromise
    • Solstice of December, and June, and Equinox of September/March are the angles to match.
      • You adjust the panels 4 times a year
Do you remember 9th-grade geometry?
To calculate the length of the stick that props up your panel is the SINE(ANGLE) x Length

Sine(angle) The angle to mount the panel.
Length is where on the panel you attach your stick.
This assumes that you are attaching your stick to the ground at a right angle, straight up to the panel.


In a spreadsheet, your equation is =sin(radians(ANGLE))*Length, the angle in degrees. Spreadsheet programs work in radians. You have to convert the degree to radian, then get the sine and multiply the length. Calculators work with degrees fine.
 
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