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Forecast "average" solar generation by month?


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Hi -

Hoping I can kick start my design and modelling with some real-life data from those who have systems up and running.

(I have my consumption info from EmonCMS probes)

I'm in Cape Town.  I'm looking at a 12x300w-ish panels mounted facing north but a bit flatter than ideal (roof pitch around 20 degrees maybe)

solarelectricityhandbook.com's insolation calculator has the following figures with panels at 71 degrees from the vertical (aka 19 degrees from the horizontal)

Cape Town
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 71° angle:
(Optimal summer settings)

 

Jan Feb Mar Apr May Jun
7.91
 
7.43
 
6.41
 
5.13
 
3.92
 
3.57
 
Jul Aug Sep Oct Nov Dec
3.75
 
4.36
 
5.49
 
6.62
 
7.53
 
7.83
 

My 12 panels are about 24m2 and 15% efficient so in "theory" in Jan 7.91x24x15% = 28kWh per day.  July 13kWh.

Now I'm sure that is just never going to happen.  I'm not going to see the full efficiency of the panels, my inverter needs a minimum string voltage to work, and what have you.

(I also looked at some local systems on pvoutput.org and I can see that the actual performance is lower)

I found this chart of "rainy days" for Cape Town (be nice, Gautengers!)

F1DBA578-8252-4DD6-A2C6-FF6238B9371A-2654-00000453DA217A2B.png.a795dfdbeac65563147182f79a382fec.png

 

So from that I might guess that average generation might be about 1/2 the theoretical in the winter, and maybe 75% in the summer?

Maybe someone has some figures I can work with?

It would also be great to know "by the hour" figures for different months.  I'm very happy to work from raw data.  I've used some estimates and it's obvious that matching demand to available power is important and tricky.

Thanks!
Elbow

 

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Why not build a frame and move the panels to a more ideal angle? Or a movable frame you can change the tilt on? The steel to do that will be less than the cost of another panel methinks.   

Most people seem able to survive with about 3kW of panels, once they have sorted out the consumption side of things and are quite frugal with electricty usage. 

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To give you an idea of the ratio, in November/December I made (scaling around the fact that I added panels in March) 150kwh a month for each installed Kwp, and in June I made 65kwh/kwp, but July then surprised me with 100kwh/kwp. So average summer (150) over average winter (80) seems to agree with your rule of thumb: About half in winter.

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3 hours ago, Elbow said:

solarelectricityhandbook.com's insolation calculator has the following figures with panels at 71 degrees from the vertical (aka 19 degrees from the horizontal)

Not sure about the accuracy of the info from the above site - 19' is very flat for your area, it will be great in summer months and poor in winter months. With Cape Town sitting at a latitude of 34' I would aim for a fixed angle of 29' above horizontal.

See the following website: http://www.solarpaneltilt.com/

... which offers the following quick calculations for fixed panels optimized for year round performance (and lots of advice for tilting)...

  • If your latitude is below 25°, use the latitude times 0.87.
  • If your latitude is between 25° and 50°, use the latitude, times 0.76, plus 3.1 degrees.
  • If your latitude is above 50°, see other situations...

If you decide to tilt your panels the additional 9 degrees from your current 20' roof remember to allow for inter-panel shading, I would recommend spacing of double the height differential - so if the top of string 1 is 200mm higher than the bottom of string 2 then spacing should be 400mm.

...otherwise lay them flat on your 20' roof and accept lower performance in winter:)

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On 8/25/2017 at 2:01 PM, pilotfish said:

If you decide to tilt your panels the additional 9 degrees from your current 20' roof remember to allow for inter-panel shading, I would recommend spacing of double the height differential - so if the top of string 1 is 200mm higher than the bottom of string 2 then spacing should be 400mm.

...otherwise lay them flat on your 20' roof and accept lower performance in winter:)

Thanks for the hints.  I'll measure the roof pitch properly when we move in and see what the actual angle is.

Steve

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Just checked my production figures and for off grid I have a solar efficiency of about 3.5 kWh/kW panels in winter up to about 4.5 in summer. Fixed 35 degree array. So for my 3kW array I am getting about 13.5kWh per day in summer, about 405kWh per month, and  10.5kWh per day in winter, about 315kWh per month.

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