You said : "I've used their data for Northeast US sites for a number of years now. With regard to insolation, they only model global horizontal insolation, however. You have to do some work to derive components you might need."
I have global radiation data too, and I would like do approximation to create an epw file. Horizontal Infrared Radiation intensity, Direct Normal Radiation and Diffuse horizontale radiation are needed.
Have you hint to help me do this job ? (link, equation...)