The National Solar Radiation Database offers solar radiation data and limited meteorological data for 237 sites in the US over the period 1961-1990, and expanded data for 1454 sites from 1991-2005:
Data includes:
- Daily Statistics Files
- Hourly Statistics
- Threshold Files
User manuals in PDF format can be downloaded. Data is in text format, but can be converted to spreadsheet format with a bit of work.