The Solar Resource

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Satellite instruments, pointing directly at the sun, measure the intensity of the sunlight above the atmosphere as 1370W/m². This is called solar insolation. In combination the earth’s clouds, aerosols, atmosphere and surface reflect an average 31% of this sunlight, leaving 69% to be absorbed.
If you calculate how much solar energy the earth (with an average radius of 6378km) intercepts, this energy dwarfs all other available energy supplies. This immense power of the sun is often denigrated by the very limited conceptions of the fleeting fossil fuel industry but what is the real situation?

Seeing as we’re bathed in far more energy from above than there is in the ground below, let’s put together the information needed to replace fossil fuel burning.
One of sunlight’s important qualities is that its rays are almost parallel (within about ½ degree of angle or 31min of arc). This allows us to concentrate the sun’s rays into a small area where the intensity can produce quite high temperatures. High temperatures permit the conversion of a greater proportion of the sun’s energy into useful work (say via generating electricity). The 2nd law of thermodynamics gives the maximum possible efficiency of conversion.

Finding the Solar energy potential at your place. An excellent resource to help determine the quantity and quality of solar energy anywhere on the Earth’s surface is: NASA Surface meteorology and Solar Energy - Available Tables, which you can access from: http://eosweb.larc.nasa.gov/sse/
For the Latitude and Longitude that you enter, this excellent database gives you a comprehensive amount of information ranging from global and diffuse insolation on horizontal and tilted surfaces, direct normal radiation plus probable variations. The tables also supply wind speeds and much other meteorological data.
For instance if you wanted to assess the amount of energy supplied to a crop in Northern Rivers, NSW, Australia over a certain period then you would use the following table. (Note that Langly provides the insolation measurements in kWh/m².day but here I have converted it to MJ/m².day.)

Insolation on horizontal surface (MJ/m².day)

Lat -28.5
Lon 153
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
10 Year Average 24.1 21.5 18.9 14.9 11.7 11.1 11.8 15.1 19.5 21.6 23.6 24.1


And suppose you wanted to size a parabolic dish for the same district then you would use the “Direct Normal” table. (Where “Normal” means perpendicular to the surface)

Direct normal irradiance / Page method (MJ/m².day)

Lat -28.5
Lon 153
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual Average
10 Year Average 22.0 19.55 18.4 16.0 14.36 15.84 16.1 18.86 21.8 20.9 21.67 21.7 18.936


To calculate the size of the energy storage needed to carry you through low sun periods there are also tables that give you the minimum radiation for consecutive periods like 1, 3, 7, 14, 21, 30 days.

What is the destiny of the unreflected incoming solar radiation and the minor contributions to the Earth’s biosphere?
The table below gives you a good idea of the relative magnitudes of the major energy paths on the earth’s surface.
Power Budget for Earth’s Biosphere. All measurements in TerraWatt (TW or 1012J/s)

Input Output
Input Source Power Output Source Power
Solar Radiation 174000TW Reflection 52000TW (30%)
    Heat 82000TW (47%)
    Evaporation & Precipitation 40000TW (23%)
    Wind, Waves & Currents 370TW (0.71%)
    Photosynthesis 40TW (0.023%)
Nuclear,Thermal, Gravitational 32.3TW Heat radiation 32.3TW (0.018%)
Tidal 3TW Heat radiation 3TW (.0017%)
Total In 174036TW Total Output 174445TW (100.7%)


Rounding errors cause the 0.73% discrepancy.

As a comparison the rate of human use of energy (extrasomatic or outside the body ie not food) is currently 12.67TW. The human population consumes food at a rate of 0.75TW.


solar radiation graph





Sunshine Energy reaching the Earth in only 24 hours compared to the total Energy in the Earth’s remaining Fossil Energy “Reserves”

solar radiation graph

Sunlight energy: Nasa satellite readings of 1370W/m2 outside the atmosphere, 30%reflected, 70% absorbed.
Fossil fuel reserves data from BP statistical review 2006. http://www.bp.com
Uranium reserve data from World Energy Council & World Nuclear Association
Humans use more than 400EJ/year of energy
World Nuclear Association http://www.world-nuclear.org/info/inf75.htm
World Energy Council http://www.worldenergy.org/wec-geis/publications/reports/ser/uranium/uranium





The Power of the Sun on the Earth


Alltogether 174000 TW is reradiated to space. 122000TW has been used in Earth processes (eg rain, wind). It goes to space as low quality longwave radiation (heat). The reflected 52000TW is high quality. The Earth intercepts 174000 TerraWatts of quality Sunshine continuously
power of the sun on earth diagram


Output Power from the Earth to Space 174457TW (rounding errors)

output power from earth to space
Scale : 1mm = 1TW

High Insolation areas of the World

The orange areas = good places to install solar energy collectors

High Insolation