Solar constant:
"The solar energy reaching the periphery of the earth's atmosphere is considered to be constant for all practical purposes, and is known as the solar constant. Because of the difficulty in achieving accurate measurements, the exact value of the solar constant is not known with certainty but is believed to be between 1,353 and 1,395 W/m² (approximately 1.4 kW/m², or 2.0 cal/cm²/min). The solar constant value is estimated on the basis of the solar radiation received on a unit area exposed perpendicularly to the rays of the sun at an average distance between the sun and the earth."
[http://almashriq.hiof.no/lebanon/600/610/614/solar-water/unesco/21-23.html]

Radius of the Earth:
The value of 6371 km used here is the Volumic Radius, ie the value giving a fairly exact value for the Earth's volume by the usual formula for a true sphere. Because the Earth is actually an oblate spheroid, other radii have been calculated.
[http://en.wikipedia.org/wiki/Earth_radius]

Calculations:
Taking the solar constant to be 1.4 kW/m2, and the Earth's radius to be 6371 km, the cross-sectional area of the Earth is pi*r^2, ie 3.142*((6371*10^3)^2)=1.2753E14 sq m.

With the Earth at its average distance from the Sun, total energy received (ignoring atmosphere etc) is 1.2753E14 times 1.4 = 1.785E14 kW. This is equal to 178,500,000 GW (gigawatts). (For a world population of ca 6 billion, solar energy per person= ca 30,000 kW.)

1 year=365.25*24 hr =8766 hr. Total solar energy received/yr = 8766*1.785E14 kW-hrs = 1.564731 * 10¹⁸ KW-hrs = 5.6330316 * 10²⁴ joules.

1 kilowatt hour = 3 600 000 joules = 3.6E6 joules. 1 kilowatt = 1.0 x 10^-6 gigawatts

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Last update 2006 Sep 18