Area Solar Radiation: how does one convert values from Wh/m^2 to W/m^2?

[WilliamHenshaw]

I have been unable to find anything online about this. The Area Solar Radiation tool (through the Spatial Analyst extension) calculates global horizontal irradiance (GHI) for each pixel, from a specified DEM layer. My research project focuses on agrivoltaic array installation, and as such, I need units to be W/m^2 so I can convert to lux. However, ArcMap's GHI equation calculates the value of each pixel in Wh/m^2. My calculation is done for the whole year of 2017. If others have had a similar experience, how did you convert the values? I'm assuming you divide each value by the number of sunlight hours in the year for the study region, but ESRI does not specify. What further bothers me is that the Natural Renewable Energy Laboratory (NREL) calculated solar potential using 8760 hours, the number of hours in a year. So I'm confused as to which value for hours is actually used for ESRI's GHI equation: 8760 or a value unique to the study region? area solar radiation

[XanderBakker · Accepted solution]

When you run the Area Solar Radiation—Help | ArcGIS Desktop tool you will have the option to generate a duration raster (hours sun) which you can use to calculate the unit you need (you can also create the optional "diffuse incoming solar radiation" and "direct incoming solar radiation" rasters). To have a look at the formulas used you can check this explanation: How solar radiation is calculated—Help | ArcGIS Desktop 

[WilliamHenshaw]

Is there any way for it to calculate duration without having to run the whole tool again? It took quite awhile to run it for my entire study region.

[XanderBakker]

I'm afraid that is not possible (at least I think it is not). However, you could "play" with the interval values, to reduce processing time, but it will yield a duration raster that might be a little less accurate.