I need your help. I'm trying to calculate the global irridiation for a specific area. I'm interested in the results for rooftops in special. I have a DEM, calculated slope, aspect and horizons and started r.sun for every day with a monthly changing Linke Factor. All calculated maps (slope, aspect, horizons) seem to have believable values. My Problem is: On the yearly sum (addition of all 365 days) I get values for total global irridiation of about 1900 KW/m². Which is 1,9times more then expected. Especially when the solar constant is only 1368 kW/m².... Could you imagine what went wrong? I'm lost.

  • Why are you summing all of 365 days? it doesn't have sense... the global year irradiation, it's the average of all the 654 days not the sum.
    – Sergio
    Sep 2, 2014 at 13:37
  • sorry, i wanted to calculate the sum of KWh/m² so, as glob_rad gives the Wh/m² per day i sum them up (and divide them by 1000)
    – Ulf
    Sep 2, 2014 at 14:38
  • is it neccesary to give input for coef_bh and coef_bf?
    – Ulf
    Sep 2, 2014 at 14:40
  • No it's not neccessary. Can you paste here the code of one day and the code that you use for calculating the sum?
    – Sergio
    Sep 2, 2014 at 14:59
  • 1
    I noticed you wanted to delete this question because you've found an answer. Instead of deleting, please consider adding the answer you've found as an answer here, and mark it as accepted, so that people with a similar issue in the future might find your answer.
    – nmpeterson
    Sep 2, 2014 at 18:21

1 Answer 1


I just found the answer fo myself, reading the manual carefully and focused. The r.sun mode 2 computes just the clear-sky irridiation. To get the "real-sky" irridiation you have to multiply the result with a clearsky Factor (Kc). Kc is defined as the ratio of messured Irridation on Ground under overcast situation and the clearsky amount messured via satellite (both for horizontal orientatian) and has values from 0-1.0 . Data to calculate the Kc-Factor can be requested here: https://eosweb.larc.nasa.gov/cgi-bin/sse/[email protected] After multiplying the results with Kc the results seem to be realistic.

Thanks all for trying to help!!!


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