YOUR GUIDE TO SOLAR ENERGY

FACTS ABOUT SUN'S RADIATED ENERGY

The single most important thing to determine when choosing your solar power system is how much electricity it will generate for your home. This requires calculating of the average amount of the energy that reaches the earth in your area. Sun of course radiates in all directions. At the surface of the Sun, the radiated energy flux density is approximately 63,000 kW per square meter. This flux drops as the square of the distance from the Sun. The yearly average Sun's irradiance at the top Earth's atmosphere on an imaginary surface perpendicular to the sun's rays is approximately 1366 watt per square meter according to National Institute of Standards and Technology NIST or 1361 W/m² according to NASA measurements. This value is called solar constant, although actually it slightly varies through 11-year cycles and is not really a constant.
You can see from the solar energy diagram shown below that the amount of solar energy intercepted by the Earth is solar constant times Earth's cross-sectional area Ac=π×d²/4, where d is the Earth diameter, π=3.14. Since the total surface area of a sphere is π×d², the amount of sunlight power per unit area that reaches our atmosphere averaged over entire Earth's surface is ¼ of solar constant, or about 341 W/sq.m.

From this amount about 29% is reflected back to space, 23% is absorbed in the atmosphere, and 48% reaches the surface.
Solar energy diagram

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Thus, an average amount of solar energy received at the Earth surface per square meter per second is 341×0.48=163 W/sq.m, or 15 watt per square foot. Note that this is the value averaged over the entire surface of the planet over a year. The closer to the equator, the more sunlight is available throughout the year. The solar power density at the equator on a bright day at noon is about 1000 watts per square meter. This value is called the "standard sun". It is used in the industry for rating efficiency and peak power output of PV panels. Outside of the equator you can probably receive this amount of energy on a surface directly facing the sun (i.e. tilted toward sun according to your latitude) at a bright noon. Remember however, this is just a peak power. During a day, the sun's radiation varies significantly as the sun moves across the sky. That is why a more important characteristic of solar radiation is the net amount of the sunlight delivered during the entire day.

SOLAR INSOLATION INFORMATION

The net amount of the sunlight received during a day varies significantly with geographical locations and the weather patterns. To calculate the average amount of electricity a residential PV system can generate, you need to know the characteristic called insolation (INcoming SOLar radiATION). Insolation levels represent an average solar energy density and are usually expressed in kilowatt-hours per square meter per day (kW-hr/m²/day), or as an amount of equivalent hours of "standard sun" 1000 W/sq.m. Numerically both values are the same. NASA maintain extensive information on average solar energy density for most locations in the world. NREL laboratory has insolation maps for the US. Note that in today's non-concentrated PV systems for homes only less then 20% of the insolation can be practically converted to electricity (see: Solar power system efficiency).

REFERENCES AND ADDITIONAL INFORMATION

U.S. solar radiation resource maps and data manual
NASA worldwide insolation data

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