US Solar Insolation Maps

US Solar Insolation Maps

Solar Insolation Maps From the National Renewable Energy Labs (NREL)

  • There are Four Maps Here:
    • Average Yearly Sun - The first one shows the yearly average
    • Best Case - The 2nd one is the "best case" or June
    • Worst Case - The 3rd one is the "worst case" or December.
    • With Tracking Mount - The 4th map shows the worst case (January) again, but with a tracking panel mount, which follows the sun.

For designing a system, you almost always use the worst case, or December-January map. "Flat-plate collector" is simply a solar panel.

The full set of maps (all 300 or so) are available at the NREL website:

Some Definitions:
kilowatt-hours per square meter: The earth at sea level receives about 1,000 Watts per square meter. If the map says 9 kWh/m2, then you are getting about 9 full hours of sunlight on the panel. Modern solar panels are around 20% efficient, so that works out to approximately 200 watts per square meter, or 20 watts per square foot.
Tilted South at Latitude: The panel is facing due South, and tilted at the same angle as the latitude. If you look at a roadmap and see the latitude is 23 degrees, then the panel would be tilted at 23 degrees.


This first map shows the yearly average, in kilowatt-hours per square meter for an average yearly day.

Translation: At high noon on a clear day, each square meter receives 1000 watts of solar power. If you look at the large yellow areas, you will see that it gets around 6,000 watts on an average day. So, even though the average day is exactly 12 hours, the power you actually get on your panels is equal to about 5 to 6 hours of full sun per day. Since the typical modern solar panel is about 19% - 21% efficient, you will get about 1200 watts per square meter of panel. So, if the map says that you live in a "six" area, you can expect sun power equal to 6 hours per day over the entire year.

Average Daily Solar USA


This map shows the yearly average for an average June (best case) day.

A large portion of the country is now yellow, showing that good solar power is available for most of the country during the summer.

June Solar Day Insolation


This map shows the yearly average for an average December (worst case) day.

As you can see, in the winter it's an entirely different story. Much of the country now gets an average of 4 hours or less of full sun hours per day.

December Solar Insolation Map USA


This map shows the yearly average for an average January (worst case) day, but with a solar tracking mount.

Compare this to the previous map, and you can see what a difference a tracking mount system can make. The example shown is for 2-axis tracker, like the WattSun. A single axis tracker, like the Zomeworks, will be a little less, but still considerably more than a fixed array. The biggest problem with tracking mounts is that they give the biggest increase in the summer, while the biggest need for power is in the winter. If you have plenty of power in the summer, but fall short in winter, your best option may be to use a maximum power point tracker (MPPT Charge Controller). Tracking mounts used to make a lot more sense when solar panels were selling in the $10+ a watt range, but with current prices around the $2.50 to $4.50 per watt range, the economic advantages of tracking are less.

Solar Insolation Map With a Panel Tracking System