The intensity of the radiations at the surface of the sun is around 6.33 x 107 W/m2 and it reduces as it spreads out into the solar system. By the time it reaches to the surface of the earth, the solar intensity is reduced to an average of 1367 watts of power per square meter in a day, some of it is reflected and scattered into the atmosphere and around 1000 Watts/m2/day amount of radiations reach the surface of the earth. This amount of radiations are more than enough to meet our global energy demand and because of this reason the global energy trend is shifting towards solar energy. The solar panels are calibrated by assuming that earth surface receives 1000 Watts/m2/day of solar insolation in clear sky.
"The total amount of solar radiations falling on the surface of the earth over a 1 square meter area in a day is termed as solar insolation & it is expressed in Watts/m2/day"
(1) Understanding the concept
Case -I: When the sun is overhead
The sunâ€™s radiations are unevenly distributed over the surface of the earth, being highest at the equator and the lowest at the poles. The earth atmosphere affects the amount of solar radiation received by the particular region or country or location. The heavy clouds and fog will restrict the solar radiation from reaching the surface of the earth. A clear sky will allow more solar radiations to reach the surface of the earth.
The intensity of the solar radiation and its duration depends on the two phenomenons,
The solar power is the maximum when sun is overhead or the radiations are perpendicular to the surface of the earth. As the radiations of the sun become more inclined with respect to the surface of the earth, their intensity starts reducing and so is the value of the solar insolation.
When the sun is overhead, the angle between the surface of the earth and the solar radiation is 90 degrees
Effective Solar Radiation = 1000 Watts/m2 x sin 90 degrees;
where, sin 90 degrees = 1
= 1000 Watts/m2 x 1
= 1000 Watts/m2
Case-II: When the sun is at 45 degrees
When the angle between the solar radiations and the surface of the earth is 45 degrees
Effective solar radiation = 1000 x sin 45 degrees
Where, sin 45 degrees = 0.707
= 1000 x 0.707
= 707 Watts/m2
(2) The intensity of the solar radiations follows the following pattern from sun rise to the sun set
Assuming 12 hours of sun in a day, the solar insolation for a day can be calculated as follows:
The solar insolation for a day = (0 + 259 + 500 +707 + 866 + 966 + 1000 + 966 + 866 + 707 + 500 + 259 + 0) Watts/m2 = 7596 Watts/m2 in a day or 7.596 Sun in a day
(3) The solar insolation value decides the size of your solar panel
The amount of solar radiations are very helpful in sizing the panels of your solar PV system. If a place or location is situated near equator where the solar radiations are more direct than the other geographical locations, the amount of solar radiations on the surface in a day will be more. The locations with more solar insolation will require smaller solar panels as compared to the areas of low solar insolation levels. For example, a location A receives a total of 5000 W/m2/day of solar insolation in a day requires smaller size of solar panels to produce same amount of electrical power as compared to that of location B which receives 4000 W/m2/day of solar insolation in a day.
Area of solar panels (m2) = Desired output electrical power from panel / Solar insolation of the location x efficiency of the panel
(4) The solar insolation levels helps in determining the feasibility of your solar roof
As you find in the above point that the lower solar insolation is associated with the large solar panels which means that extra cost is added to produce the desired output electrical power. The extra cost increases the payback period of the solar roof which ultimately affects the feasibility of the solar system. Therefore, a location with higher solar insolation contributes in feasibility of the solar roof.
(5) A few comparisons of solar Insolation levels of India with other countries:
The solar insolation is a measure of the amount of radiations on the specific surface area of the earth in a given time period. The solar insolation time period is generally taken as one day. In a clear sky, on an average of 1000 watts of sun energy is received by the earth in square meter area. A 1m2 panel (efficiency of 15%) is capable of illuminating 150 watts of electrical bulb at peak sun.
The amount of solar radiations reaching to the surface of the earth depends on the angle of the sun with respect to the surface of the earth, altitude, the atmospheric conditions and the geographical location.