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The Renewable Energy site for Do-It-Yourselfers
Heating Degree-Day map of US from the National Climatic Data Center website: http://lwf.ncdc.noaa.gov/oa/ncdc.html
More detailed maps are downloadable here on the NCDC site:
http://lwf.ncdc.noaa.gov/oa/documentlibrary/clim81supp3/clim81.html