There is no dramatic drop-off in solar potential in the continental U.S. as you move away from the desert Southwest—as you might suppose from the colors of the federal map at left.

To help promote understanding of solar’s potential, the National Renewable Energy Laboratory (NREL), which created the map at left, could discontinue use of a misleading color scheme.

For example, data from the key, at the bottom right of the map, show that global horizontal solar irradiance in the two northern bands is 70% of that in the two sunniest Southwest bands (4.00 kWh/m2/day versus 5.75 kWh/m2/day).  That’s indicated by the moderately intense red colors at the top of the map at right—prepared by the author—but the pale yellows at the top of the map at left are nowhere close to 70% of the intensity of the map’s colors for the Southwest.

NREL’s solar irradiance maps date back at least as far as 2009, when dramatizing the most promising regions for solar may have been appropriate—because back then, solar was expensive, and maximizing the revenue from every solar panel was important.  But now that solar is inexpensive, it’s useful to see at a glance what solar’s potential is in every part of the country.

The map at right starts with pure red for the Southwestern band with the highest irradiance.  The map incrementally adds more white, to create a less intense shade of red, for each lower-irradiance band, on a scale where zero irradiance would be represented by pure white. Because even the northernmost two bands have 70% of the irradiance of the far Southwestern band, their colors are pinkish reds that are about 70% as intense as pure red.  And generally, the map appears red, because there’s substantial solar potential everywhere.

Global horizontal solar irradiance counts all solar radiation reaching the earth, whether directly from the sun or after passing through clouds.

Technical note

To create the map at the right, the map at left was first imported into Photoshop. The original map’s deep violet red in the lower left was replaced with pure red, specified in Photoshop as R = 255, G = 0, B = 0 (RGB stands for red, green, blue).

This pure red color was assumed to represent 5.87 kWh/m2/day, because the key defines this band as having global horizontal solar irradiance greater than or equal to 5.75 kWh/m2/day, and the other bands represent a range of 0.25 kWh/m2/day. Thus, 5.87 was used as the denominator in determining the ratio of any other band’s irradiance, to this “maximum” band’s irradiance.

The numerator for a given band’s ratio was set as the numerical midpoint for the band.  For example, for the band next to the “maximum” band, 5.62 is the approximate midpoint of the range of 5.50 to 5.75.  Thus, the ratio of the second band’s irradiance to the first band’s irradiance is 5.62/5.87, or 96%. In Photoshop, to select a shade of red that is 96% as intense as pure red, a color was selected that is 4% of the way from pure red to pure white. That color happens to be R = 255, G = 11, B = 11.  That’s because pure white is R = 255, G = 255, and B = 255, and 11/255 is about 4%.

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