Every moment Sun releases 3.8 x 10^26 Joules of energy in to our solar system.
â€‹But earth receives small fraction of that energy on its surface.
Most of it is reflected back and absorbed by earth's atmosphere before striking the ground.
By the time, it reaches the earth's atmosphere, the solar intensity is reduced to 1367 watts/mÂ².
Again, some of the energy is reflected and scattered into the atmosphere, and finally 1000 watts/mÂ² strike the ground at noon when the sky is clear.
when you tap this energy, it can meet our global energy demands.
Read: why global energy trend is shifting towards solar energy?
The value â€‹of 1000 watts/mÂ² is called One Peak Sun Hour (PSH).
The power rating (Pmax) of the solar panels are calibrated by assuming that the earth surface receives 1000 watts/mÂ² of Peak Sun Hours in a clear sky.
(*The other standard conditions are: cell temperature 25Â° and air-mass index 1.5).
The total sunlight falling on the earth surface over 1mÂ² area in a day is termed as solar insolation and it is expressed in watts/mÂ²/day
Variation in Peak Sun Hours (PSH)
The sunâ€™s radiations are unevenly distributed over the surface of the earth, being highest at the equator and the lowest at the poles.
â€‹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 following factors:
A place on the earth will receive more solar insolation, if the solar radiations are falling more directly at the surface and for longer duration.
The sun rays are more oblique at the poles and more direct around the equator. The more value of solar insolation means that particular area is receiving more solar radiation per unit area per day and hence, making that region more feasible for installing solar roof.
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.
Case -I: When the sun is overhead
When the sun is overhead, the angle between the surface of the earth and the solar radiation is 90 degrees
Effective Solar Radiation
\(=1000 * 1\) as\((sin90^o=1)\)
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
(2) The intensity of the solar radiations
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) \(Watts/m^2\) = 7596 \(Watts/m^2\) 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.
â€‹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 \(Watts/m^2/day\) 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\(Watts/m^2/day\) of solar insolation in a day.
(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.
â€‹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.
â€‹In a clear sky, on an average of 1000 watts of sun energy is received by the earth in \(1m^2\) of area. This 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.