As outlined in this link ( r.sun delievering unrealistic Values ) to get realistic values of kilowatt / meter-2, you must multiply the results by a "clear sky factor (Kc)". The link comments recommend this website, https://email@example.com, but there is no option to determining Kc. I have tried to follow the directions on determining the ratio of overcast situation to clear sky but neither are options on the NASA page. Also, I don't know how to multiply the output raster by whatever the Kc value might be. Can anyone assist?
From the original answer:
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://firstname.lastname@example.org After multiplying the results with Kc the results seem to be realistic.