Where the Sun Shines

Solar power plants require solar energy.  If the sun doesn’t shine, there is no solar energy.  If the sun is obscured, there is no solar energy.  If the angle of the sun striking a surface is increased, there is less solar energy.  If the sun is reflected or scattered from a surface, solar energy is diminished.  All things considered, solar energy is the driving force for solar power plants, and availability of solar energy is location dependent. [i]


Location, Location, Location

Every location on Earth receives sunlight at least part of the year, some areas more than others.  The amount of solar radiation that reaches any one “spot” on the Earth’s surface varies according to the following factors:

  • Time of day
  • Geographic location
  • Season
  • Local landscape
  • Local weather

Extreme latitudes on the Earth’s surface can put a strangle hold on solar energy.  Due to the curvature of the Earth, the more northern, southern, and polar regions receive less direct sunlight than the lower latitudes, and to add insult to injury, the polar regions receive no sunlight at various times of the year.  For this reason, higher and lower latitudes are poor locations to harness solar energy.

The Earth rotates around the sun in an elliptical orbit which impacts proximity to the sun.  In the summer, the southern hemisphere is closer to the sun than other times of the year.  In the winter, the northern hemisphere is closer to the sun.  This has minimal impact on solar energy.

Seasonal changes affect length of day.  Days are longest in the northern hemisphere between the spring and fall equinox.  In the southern hemisphere, the longer days are between the fall and spring equinox.  Day and night during each equinox are exactly 12-hours long.  Seasons and day length significantly affect solar energy.  See “Chart of Day Lengths and Time of Year at Different Latitudes.”


Chart of Day Lengths and Time of Year at Different Latitudes


Middle latitude countries, such as the United States, receive more solar energy in the summer not only because the days are longer but the sun is nearly overhead.   In the more northern states, solar measurements over a summer day have historically received 4 to 6 times more solar energy than that received in early winter days. In the more southern states the differences between summer and winter solar energy is 2 to 3 times greater in the summer than in the winter.[ii]



Insolation, Not Insulation[iii],[iv],[v]

The amount of solar energy falling on a specific geographic location is referred to as insolation—or “incident solar radiation.”  On a clear day with the sun directly overhead, the most solar energy that reaches a surface on Earth is 1,000 W/m2.  This maximum level has been adopted by the solar energy industry as a standard for quoting “peak power” and “installed capacity” for solar photovoltaic panels—measured in terms of kWh/m2/day.

The “actual energy” available is based on location, not on the rated capacity.  This is a very important distinction.  Once again, rated capacity and actual solar energy available at a location are not one and the same.

Each geographic location on Earth will have a different set of insolation values—both averaged and seasonal.  As you would expect, locations closer to the Equator receive greater solar radiation than at the North/South Poles.  The greater the latitude, the less solar energy reaches the surface, the less the insolation number.  In Anchorage, Alaska, at about 61o north latitude, the average annual insolation is 2.09 kWh/m2/day, and in Miami, Florida, at about 25o north latitude, the average value is 5.26 kWh/m2/day.  There is considerably more solar energy available closer to the Equator than at the extremes.

Insolation also varies with the seasons. As it rotates around the sun, the earth’s axis shifts, causing the sun’s position to seasonally migrate relative to latitude.  Change in the position of the sun has an effect on the intensity of the solar radiation and length of day.  The greater the angle to the sunlight, the greater the reflection and the less the energy strikes a surface.  In the northern hemisphere, the greatest insolation values are during the Summer Solstice (June), and the least are during the Winter Solstice (December).  They reflect the longest and shortest days, respectively.

Well, up to the point where you start looking at insolation maps, the logic is great.  Now, look at an insolation map!  It appears as though the rules are not applicable in the real world.  There seem to be other influences at play—altitude and frequency of cloud cover.  There is more energy available at mountain elevations than at sea level—due to shortened distances for the radiation to travel.  The areas on the U.S. Insolation Map that seem to show high insolation levels for their latitude are regions that have the highest elevations in the United States.  In the western mountain states, the highest elevation is in Colorado at 14,440 feet.[vi]  The small mountain region along the eastern portion of the New England states has less altitude.

High frequency, long-term cloud cover or fog will result in diminished solar radiation intensity. Looking at an insolation map of the world, the most striking observation is that within 15o of the Equator tend to have lower insolation measurements than less arid regions between 15o and 35o north and south of the Equator. The reason for this is that the hot equatorial regions tend to have high humidity with extensive cloud cover.


Insolation Map of United States


In the United States, average insolation levels range from 1.1 kWh/m2/day in the north Washington area to 5.5 kWh/m2/day in the southwest portion of New Mexico.  In Europe, the insolation levels average 94 W/m2/day in Edinburgh, England to 215 W/m2/day in Limassol, Greece. The seasonal variations are 13 to 172 W/m2/day and 96 to 325 W/m2/day, respectively.  Typically, these numbers are used to determine how many solar panels will be needed.  Five times more panels would be required in the north Washington area, 25 times more in Greece, and 58 times more in England than that which would be required in the southwest section of New Mexico.  These numbers are staggering.

Worldwide, the greatest values are on the African continent in the Sahara desert and a large portion of southern Africa areas; a large portion of Australia; and a small portion of South America (greater than 6.0 kWh/day).  Most worldwide locations greater that 60o latitude receive less than 1.0 kWh/day.[vii]


World Insolation Map


Some of the insolation maps are reported in terms of sunlight hours/day.  Oh, now, let’s take a look at this moment of confusion.  Remember, the most solar radiation that can reach Earth is 1,000 Wh/m2/day.  This is the highest possible insolation number—also referred to as a “sun unit.”  In other words, 1 sun unit is equivalent to 1 sunlight hour/day.  Still sounds a bit dicey?  Let’s look a little closer:

A location receives sunlight 10 hours a day in the winter, but the amount of solar energy hitting the surface at that location totals 3,500 Wh/m2 –over the entire day. The location may receive 20 Wh/m2 at sunrise with radiation increasing to over 600 at the peak of the day, falling off at nighttime. But the total of the irradiance measured throughout the 10 hour day adds up to 3,500 Wh/m2/day.  Remember a sun unit is 1,000 Wh/m2/day.  Divide 3,500 Wh/m2 by 1,000 Wh/m2/day, and the magic number is 3.5. So, there were 3.5 kWh/m2/day or 3.5 sunlight hours striking the Earth’s surface on that winter day.  See this concept in graph form on Figure 3-7.


 Insolation Interpreted into Sunlight Hours/Day[viii]


The use of sunlight hours/day makes it easier to understand and to crunch numbers—should you so choose.  You will have this opportunity in the Section 4, “Home Owners Beware!”

Then, there are the solar thermal power plants. They too require solar radiation but they require it in the form of heat.  Dessert heat, a mirage of energy, is best suited for solar thermal plants—particularly where there is greater insolation.  An ideal site for solar thermal power plants is the Sahara Dessert.  Thus, desert regions where there is high solar irradiance (e.g., insolation values) are target rich areas for solar thermal power plants.


Not to Be Overlooked

The solar power business is alive-and-well in photovoltaic panels, whereas solar thermal power is hiding in the weeds.  They are two entirely different technologies, two entirely different sets of problems.




[i] U.S. Department of Energy:  Solar Radiation Basics.  Energy Efficiency and Renewable Energy, U.S. Department of Energy. http://www.energysavers.gov/renewable_energy/solar/index.cfm/mytopic=50012?

[ii] Discover Solar Energy:  What is “Solar Radiation”? (updated:  7/7/08).  http://www.discoversolarenergy.com/solar/radiation.htm

[iii]Wikipedia:  Insolation. Viewed on 12/8/2009 at http://en.wikipedia.org/wiki/Insulation

[iv] Midwiny, Dr. Michael:  Physical Geography.net—Chapter 6. Energy and Matter (2/7/2009. Viewed on 12/8/2009 at http://www.physicalgeography.net/fundamentals/6i.html

[v] Jensen, Terry:  Insolation—Alternative Energies for a Green Home or Business.  Viewed 12/8/2009 at http://dfwnetmall.com/energy/insolation-solar-radiation.html

[vi] Wikipedia:  Elevation Data. Viewed on 12/9/2009 at http://en.wikipedia.org/wiki/List_of_U.S._states_by_elevation

[vii] altE University:  Solar Insolation Map.  http://howto.altestore.com/Reference-Materials/Solar-Insolation-Map-World/a43

[viii] Honsberg, Christina and Stuart Bowden:  Photovoltaics CDROM.  2.26 Average Monthly solar Radiation.  Viewed on 12/9/2009 at http://pvcdrom.pveducation.org/index.html