Top reasons for the variation in Peak Sun Hours

1 Peak Sun Hour = 1000 watts/m²

The total sunlight falling on the earth’s surface over 1m² area in a day is termed solar insolation and it is expressed in watts/m²/day.

When you divide solar insolation by 1000, you get the value of Peak Sun Hours for that day.

This value is not constant.

And

In this post, I will discuss the main reasons for the variation in Peak Sun Hours.

Before that, let us understand the basics of Peak Sun Hours.

Peak Sun Hours

Every moment Sun releases 3.8 x 10²⁶ Joules of energy into our solar system.

But earth receives a small fraction of that energy on its surface.

Most of it is reflected and absorbed by the earth’s atmosphere before striking the ground.

By the time, it reaches the earth’s atmosphere, the solar intensity is reduced to 1367 watts/m².

Our earth receives two types of radiation on its surface:

Direct and diffused.

Direct sunlight strikes the ground without reflection while the diffused energy reaches the earth’s surface after reflecting from the atmosphere, scattered by the clouds, smog, dust, and other particles in the air.

And

Finally, 1000 watts/ m² strike the ground at noon when the sky is clear.

And

when we add both direct and diffuse radiations from sunrise to sunset, we get total solar radiations of the day.

The value of 1000 watts/m² is called One Peak Sun Hour (PSH).

An example to find it

For example, the solar radiations or solar insolation of a place every hour are:

The total solar radiations are numerically equal to the Peak Sun Hours in a day.

The solar panel power rating (Pmax) is calibrated by assuming that the earth’s surface receives 1000 watts/m² of Peak Sun Hours in a clear sky.

(*The other standard conditions are: cell temperature 25° and air-mass index is 1.5)

Top reasons for the variation in Peak Sun Hours

The sun’s radiations are uneven over the surface of the earth.

The shape of the earth and the location:

The shape of the earth and location is the main reason for the variation in Peak Sun Hours.

​Due to the earth’s spherical shape, the sunlight becomes oblique and takes a longer path when we go from the equator towards the poles.
 
Due to this reason, the sunlight intensity is maximum at the equator, starts reducing as we go farther, and it is minimum at the poles.

Earth’s atmosphere and climate:

The earth’s atmosphere affects the solar radiation received by the region or country.
 
A dry place receives more sunlight than the humid one. Any place near to the sea and other water bodies is more humid.

The water droplets in its atmosphere trap the sunlight and prevent it from reaching the ground.
 
Therefore, people living in these regions should look for efficient, salt and wet resistant solar panels for their solar power systems.
 
Also,
 
The Peak Sun Hours is less in places that are far from the equator or at high altitudes.
 
And
 
Snowfall is a common phenomenon in these regions.
 
Solar panels with a high snow load rating are suitable in these regions.

Rotation of the earth

Earth’s axis of rotation is tilted by 23.5° with the vertical.
 
It takes 24 hours to rotate about its axis and we call it 1 day.

This rotation makes day and night on earth and creates variation in PSH from sunrise to sunset.

The sunlight intensity is minimum during sunrise and sunset while it is maximum at noon when the sun is overhead.

The solar insolation at any time in a day is calculated as:

I = I₀ cosØ

Where Ø is the angle between the actual position of the sun with its zenith point (the highest point when the sun is overhead).

During sunrise and sunset, the sunrays graze horizontally and make 90ᵒ with the zenith point.
 
Here, the sunlight intensity is zero
 
I = I₀ 90ᵒ
cos90ᵒ = 0
I = 0 (minimum)
 
And
 
At noon, the sun aligns with the zenith point and the angle between the two is zero.
I = I₀ cos0ᵒ
cos0ᵒ = 1
I = I₀ (maximum)
 
From sunrise to noon, the sunlight intensity increases, and thereafter till sunset, it decreases.
 
The total solar insolation or PSH in a day is calculated by adding sunlight values every hour from sunrise to sunset.

Revolution of the earth:

​Earth in addition to rotating around its axis, it is revolving around the sun.
 
And
 
Its orbit around the sun is elliptical (not a perfect circle).
 
When moving in an elliptical orbit, the earth’s distance from the center of the sun keeps changing.

When it approaches the sun, the sunlight intensity starts increasing
 
And
 
The intensity reduces when it recedes from the sun.
 
When the distance between the sun and the earth is the minimum, we see summers and sunlight intensity is the maximum on the earth’s surface.
 
When the earth is the farthest from the sun, winters come and the sunlight intensity is the minimum.
 
Seasons are the phenomenon of the revolution of the earth.
 
Hence, we see the variation in Peak Sun Hours.

Sun Peak Hours decide the size of the solar panels

The number of solar radiations is very helpful in sizing the panels of your solar PV system.

A person living in a sunshine place will need fewer solar panels to meet her/his energy needs than another person with the same energy needs, living in a sunlight deficit region.

For example, two persons A and B with the same energy needs of 12 kWhr units per day, living in different locations with PSH of 4 and 6 respectively.
​
(Ignoring all the losses)
Person A: Panel size: 12 units/4 = 3 kW
Person B panel size: 12 units/6 = 2 kW

Good Sun Peak Hours improves the financial feasibility

The solar power system is an investment and its return depends on the cost.

Solar panels are the major cost component in any system.

And

With good sunlight, you can meet your energy needs with fewer panels.

And

Fewer solar panels mean reduced system costs.

Now, you have faster payback and improved annual returns from the system.

​Therefore, good sunlight improves the financial feasibility of the solar power system.

Solar Feasibility Spreadsheet is an amazing tool to design and find the financial feasibility of the solar power system

Best for homeowners, solar energy students, and newbie solar entrepreneurs

A few comparisons of sunlight intensity values of different countries

Conclusion

Solar insolation is a measure of the number of radiations on the specific surface area of the earth in a given time.

The solar insolation time is generally taken as one day.

In summers at noon when the sky is clear, 1000 watts of solar energy is received by the earth in a 1m² area. 

Finally, we know that the Peak Sun Hours in any region depend on:

• Rotation
• Revolution
• Climate (humid or hot)
• latitude