Monthly 30-year "normal" dataset covering the conterminous U.S., averaged over the climatological period 1991-2020. Contains spatially gridded average annual daily total clear-sky global shortwave radiation on a horizontal plane at 4km grid cell resolution. Clear sky radiation was modeled with the Image Processing Workbench (IPW) and MERRA-2 satellite data. Distribution of effective cloud transmittance point measurements to the spatial grid was accomplished using the PRISM model. Total shortwave radiation on horizontal and sloped ground surfaces was modeled with IPW, using the clear-sky and transmittance grids as inputs. Terrain data was derived from a 30-arc-sec version of the US National Elevation Database. Modeled values incorporate the effects of elevation, shading, and reflection from nearby terrain on solar radiation. This dataset is available free-of-charge on the PRISM website.