It appears the US NREL has made an API available for this data. However, it is monthly averages, rather than live data by date. I think for this data, that's just how it is. It's also not clear to me what exactly the spatial extent is.
Let's write a WL function to query the data. You should replace DEMO_KEY with your own key after signing up, or you will very quickly hit rate limits. Signing up is free: https://developer.nrel.gov/signup/
solarData[place_] :=
Import[URLBuild[
"https://developer.nrel.gov/api/solar/solar_resource/v1.json", <|
"api_key" -> "DEMO_KEY",
"lat" -> QuantityMagnitude@Latitude@place,
"lon" -> QuantityMagnitude@Longitude@place|>], "RawJSON"][
"outputs"]
Which we can then use with:
solarData[Entity["AdministrativeDivision", {"Kansas", "UnitedStates"}]]
<|"avg_dni" -> <|"annual" -> 5.32,
"monthly" -> <|"jan" -> 4.48, "feb" -> 4.72, "mar" -> 4.87,
"apr" -> 5.47, "may" -> 5.86, "jun" -> 6.23, "jul" -> 6.67,
"aug" -> 6.14, "sep" -> 5.78, "oct" -> 4.91, "nov" -> 4.58,
"dec" -> 4.03|>|>,
"avg_ghi" -> <|"annual" -> 4.62,
"monthly" -> <|"jan" -> 2.47, "feb" -> 3.26, "mar" -> 4.27,
"apr" -> 5.49, "may" -> 6.42, "jun" -> 6.79, "jul" -> 6.86,
"aug" -> 6.13, "sep" -> 5.04, "oct" -> 3.67, "nov" -> 2.73,
"dec" -> 2.16|>|>,
"avg_lat_tilt" -> <|"annual" -> 5.44,
"monthly" -> <|"jan" -> 4.46, "feb" -> 4.91, "mar" -> 5.31,
"apr" -> 5.84, "may" -> 6.05, "jun" -> 6.08, "jul" -> 6.31,
"aug" -> 6.28, "sep" -> 6.01, "oct" -> 5.22, "nov" -> 4.69,
"dec" -> 4.07|>|>|>
Another option would be to download data from https://globalsolaratlas.info/downloads/world or some other source (there are a few) and index into it locally, but this is a bit more hassle. In fact, if I was to use the Global Solar Atlas data, I would probably load it into a local Postgis database (using raster2pgsql) and query it that way, since Wolfram Language's ability to query into large spatial datasets is not as good as Postgis' is. Maybe someday :)