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Discuss Does roof angle majorly affect perfomance? in the Solar PV Forum area at ElectriciansForums.net

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GaryM

Manchester area, just before noon on a bright cold sunny day.
South facing, roof angle 28 degree's, 4kWp system comprising of 16 x 250W JA Solar Mono panels and a Fronius IG TL 3.6 inverter.
Currently generating around 2.96kWp with a total of 7.5kW as we stand at 11:51

Will the low angle of 28 degrees reduce performance with the low winter sun?
 
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GaryM

Might have been a little premature as it's now peaking at 3.20kWp so looking pretty good. Still wondering if roof angle plays a major factor though.
 

whinmoor

Electrician's Arms
I believe a pitch of 40 degrees will be better in winter and 30 degrees will work better in summer. Over the course of the year they pretty much balance out and there's not much you can do about it anyway (unless your array's mounted on a flat surface where you can adjust the pitch to optimum tilt) so you just have to live with it.
 

FB.

Electrician's Arms
For South-facing........
In winter, roof angles around 70 degrees generate more - but lose out in summer as the sun is so high that it almost shines onto the edge of the panels.
Similarly; in summer, roof angles around 30 degrees generate more - but lose out in winter to the low sun shining almost along their edges.

Considering that generation is greatest in summer but some happens in the other seasons of the years, the ideal angle for maximum yearly generation is about 37 degrees for South-facing.

For systems which face East or West, the closer to a flat roof (horizontal; 0-degree angle) the better, since this puts the panels in a better position to catch summer sun.

For SouthEast or SouthWest, the optimum angle is about 31 degrees.

A few degrees to several degrees either side of optimum doesn't make much difference.

However, the considerable variation in regional weather patterns can make a mockery of this theoretical optimum.
Where I live (East Anglia), we tend to have an unusually large number of bright sunny mornings which become hazy/cloudy in the afternoons. The abundance of morning sun would therefore mean that slightly better performance would probably be achieved in my area by a slightly steeper (than theoretical optimum) SouthEast roof, or by a slightly flatter (than theoretical optimum) SouthWest roof.
Bearing in mind the sunny mornings here, a roof facing slightly East of South (SouthSouthEast) might well get the highest annual solar yield in my area - especially since the panels are cool in the mornings. The temperature peaks around 2.30pm and panel efficiency drops with higher temperature.
 

FB.

Electrician's Arms
I would think that with shallower roof angles, there'd be more chance of degraded performance due to hot air becoming trapped under the panels and not a steep enough angle for rain to wash off the dust as effectively.
Again, local climate/rainfall could easily ruin the theory.
 
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GaryM

Thanks lads.

In the end today my system peaked around 3.2kWp and generated 18.45kWh so not too shabby really. Looking forward to long sunny days now!
 

Jacobbb

EF Member
I would think that with shallower roof angles, there'd be more chance of degraded performance due to hot air becoming trapped under the panels and not a steep enough angle for rain to wash off the dust as effectively.
Again, local climate/rainfall could easily ruin the theory.
We have a 4kw, (16x 250w LG panels) with a 4000TL. SW facing roof. It generated 15units today with a peak of around 3kwh. Does this sound about right? Was a nice day today, along with the cold breeze.
 

FB.

Electrician's Arms
We have a 4kw, (16x 250w LG panels) with a 4000TL. SW facing roof. It generated 15units today with a peak of around 3kwh. Does this sound about right? Was a nice day today, along with the cold breeze.
Yes, 15kWh from a 4kWp SW-facing sounds about right.
I generated 14kWh from my 3.75kWp SE-facing system today, with near-full sunshine all day. My system peaked at about 2.8kW around 10.30am (you'd probably peak around 1.30pm with your SW facing).
 

danesol

Regular EF Member
Yes, 15kWh from a 4kWp SW-facing sounds about right.
I generated 14kWh from my 3.75kWp SE-facing system today, with near-full sunshine all day. My system peaked at about 2.8kW around 10.30am (you'd probably peak around 1.30pm with your SW facing).
I generated 9.33kw from a 2.88kwp S facing 30degree pitch array mostly between 10am and 2pm peaking at noon'ish ( shaded till 10am )
 
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jerriais

I would think that with shallower roof angles, there'd be more chance of degraded performance due to hot air becoming trapped under the panels and not a steep enough angle for rain to wash off the dust as effectively.
Again, local climate/rainfall could easily ruin the theory.
This is correct. Most panel manufacturers recommend that the panels are on at least a ten degree slope so the rain can do a decent job of cleaning them.
 

langstroth2

Regular EF Member
Using the pvgis tool (PV potential estimation utility), you can get a good idea of the effect of pitch by changing the pitch value for the same location/array size etc.

e.g.
4kWp array. S.England. South Facing. 14% losses. 20 degrees. Total for the year: 3800kwH
4kWp array. S.England. South Facing. 14% losses. 45 degrees. Total for the year: 3860kwH
4kWp array. S.England. South Facing. 14% losses. 60 degrees. Total for the year: 3660kwH
 
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FB.

Electrician's Arms
Using the pvgis tool (PV potential estimation utility), you can get a good idea of the effect of pitch by changing the pitch value for the same location/array size etc.

e.g.
4kWp array. S.England. South Facing. 14% losses. 20 degrees. Total for the year: 3800kwH
4kWp array. S.England. South Facing. 14% losses. 45 degrees. Total for the year: 3860kwH
4kWp array. S.England. South Facing. 14% losses. 60 degrees. Total for the year: 3660kwH
And that nicely demonstrates how most roof angles only have a moderate effect on output, just as total anual yield is only moderately affected by the direction the panels face.
Only at the extremes near to flat/vertical mounting, or East/West facing, does the effect of angle/facing become serious.
 

langstroth2

Regular EF Member
And that nicely demonstrates how most roof angles only have a moderate effect on output,
And how which angle is "best" changes throughout the year:

e.g.
4kWp array. S.England. South Facing. 14% losses. 20 degrees. Dec: 102kwH; June: 510kwH
4kWp array. S.England. South Facing. 14% losses. 45 degrees. Dec: 136kwH; June: 466kwH
4kWp array. S.England. South Facing. 14% losses. 60 degrees. Dec: 145kwH; June: 413kwH

Roll on summer (for someone with a 20 degree pitched roof)
 
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