Effect of Roof Pitch

[iantowil]

Had my Sanyo 4KWp/Sunnyboy 4000TL fitted mid July and got nearly 28KWh on the 3rd day. Now struggling to get 20KWh on a near perfect day yesterday (could have done with more wind to cool the panels). I'm assuming the shallow angle of my roof - about 20 degrees - is the reason. Are others with a shallow angle also seeing a big drop off as the sun gets lower in the sky? A friend at work with the same system is producing 2-3KWh more but has a roof pitch of 35degrees, and we are both directly South.

Also seeing my maximum power peak down to about 3KW from about 3.7KW earlier in the year. Are others with shallow roof angles seeing the same?

[billt]

It'll affect everyone, it's generally known as the days getting shorter!

The peak power will be reduced because the panels are warm.

FYI, my 3.87 kW system at 40 degrees managed just under 26kWhr on the 24th of July and 20.5 kWhr yesterday.

If you want to know what effect panel angle has on your system got to the PVGIS site and try different angles in the calculator.

http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php#

[iantowil]

Yes, I had noticed! And yes, I've read about the temperature coefficient. And the roof angle, but was wondering what others with a shallow roof angle were seeing?

Oh yes, also done the PVGIS - about 4% worse at 20degree compared to 35degree - thanks for posting the link though 

[M]

Had my Sanyo 4KWp/Sunnyboy 4000TL fitted mid July and got nearly 28KWh on the 3rd day. Now struggling to get 20KWh on a near perfect day yesterday (could have done with more wind to cool the panels). I'm assuming the shallow angle of my roof - about 20 degrees - is the reason. Are others with a shallow angle also seeing a big drop off as the sun gets lower in the sky? A friend at work with the same system is producing 2-3KWh more but has a roof pitch of 35degrees, and we are both directly South.

Also seeing my maximum power peak down to about 3KW from about 3.7KW earlier in the year. Are others with shallow roof angles seeing the same?

I've got a 2.4KWp system and a 1.2KWp system, both on same alignment, but the smaller system is on lower roof and is at 20 degrees , whereas the 2.4 is at 30 degrees. In August the smaller system was producing very close to half of the larger, now it is only about 40%. Hopefully Phase 2 (the little system) will make up ground during May, June and July. Or possibly not, ho hum.

Martyn.

[iantowil]

Thanks Martyn - just the info I was after and backs up what I'm seeing against my pals. I'm gonna give them a wash tomorrow - just to make sure. Sad or what? 

[AlanIOW]

Hi

Yes, your shallow pitch is beginning to have a detrimental effect, mine is 18 dgrees pitch and I have only managed 17-18 kWh on each of the last 2 completly sunny days, whereas I would get 28ish in mid summer. The sky has been a bit misty here on the south coast and according to our local weather station the sun's irradiance peaked at 600w/m2 today as against 1000 - 1100w/m2 mid summer, which explains why your peak generation has reduced. I cleaned my panels this morning to see if it made any difference, output did improve by about 5%for a few minutes due to the cooling effect of the hose, but it returned to normal within 10 minutes. You can see a blip on the graph at about 11.00am on my live output here;

http://pvoutput.org/intraday.jsp?id=2476&sid=1872&dt=20110929

Just seen you are planning to wash them tomorrow, great minds think alike, even though it did not help!!

[iantowil]

Alan, that's excellent thanks - exactly what I'm seeing. I like the blip from the cooling 

Funnily enough my pal tried cooling his with a hose just after having his system - bit of fun on a Friday afternoon - and he was chasing 30KWh from a 4KWp system. However, nearly ended up in casualty as he burnt his feet on the roof - water wasn't much good as most of it went over his feet. Did admire his committment in chasing what has become our holy grail 

[AlanIOW]

I rekon 30kWh in one day is possible from a 4kWp system, but it will have to be near June 21st on a perfectly clear and cool blue sky day. My best so far is 27.81kWh on 3rd June this year.

[volzhskyboy]

My 4kWp system 'Up-Norf' went live on the 8th July and managed 27.7 kWh on 14th July so I'm sure a similar sized array in the 'Costa-Del-South' should have no problems topping 30kWh in a day 

[rogeriko]

The panels have to be at 90 degrees to the sun to capture the full radiation per sq meter. As the angle gets shallower its as if the panels are smaller therefore less power. The panel has to actually present the largest possible area to the sun to catch all the irradiation. Take a sheet of paper and watch the shadow get bigger and smaller as you twist the paper in relation to the light source.

[volzhskyboy]

The PVGIS calculations suggest that May can be of 11.5% greater potential output than July for my system size/orientation, so, based on that it should be feasible for the 27.7 kWh noted for a day in July to be realised as a potential 30.9 kWh days output in May, I'll let you know in 8 months time!!!

[marshman]

I rekon 30kWh in one day is possible from a 4kWp system, but it will have to be near June 21st on a perfectly clear and cool blue sky day. My best so far is 27.81kWh on 3rd June this year.

Should easily be achievable with a good 4kW system!  I got:

 24.5  25/5/11
 25     3/6/11
 25.1  4/7/11     (best day ever)

and that is on a 3.15kWpk system through a SB3000 inverter  so a 4kWpk system should be around 25% better!

Roger

[SteveH]

 I'm not sure what these online calculators take into account when projecting output...

 I suspect they are heavy on geometry & maths, & weak on physics meteorology & will always be short on local topography. I'm probably being a little simplistic, but then I am...

 So less daylight hours will mean less power produced, a lower sun will mean more refraction through a deeper atmosphere & as a result less intensity of solar radiation. The lower sun will also means less sunlight will be able to break through any gaps in the clouds because of the more oblique angle, as when compared with a cloud scattered  summer sky.

 if your in any location close to the sea you will get more solar irradiation, up to 20% in the south east of the UK. This is in part due to more cloud forming (condensing) over the land. coastal locations also probably have lower horizons than most inland areas. Fewer coastal locations have high vegetation to take into account in the winter...

 I take the calculator as a very rough guide....

 

[AlanIOW]

Should easily be achievable with a good 4kW system! 

Roger

I agree 30kWh should be easily obtainable, but having just had a quick look through the 164 systems registered on PVOutput.org, the highest daily total I can find is 29.172kWh which is a 4kWp system near Liverpool on 14/7/11. There is a 9.88Kwp system on there which did 67.321kWh on 2/7/11, but per kWp that is not as good.

So, has anyone on here had more than 30kWh from a sub 4kWp system??

[iantowil]

My Pal I mentioned above that does the firewalking on his roof got pretty close - 29.8KWh. Wasn't happy that his wife hadn't been spraying them with water through the day  - what can you do 

I had my system on the 12th July and had a virtually identical best day to Volzhskyboy at 27.7KWh (I read he had his a few days before me), with an identical system - only the roof pitch is different - and I'm in "Sunny" South Wales, ahem!

These PV systems can send you slightly mad   

Marshman - your system/location seems to be the best by some way - I think you'd smash 30KWh if you had 4KWp system.