Solar irradiance is the amount of sunshine incident on a unit area and is typically expressed in watts per square meter (W/m2) or kilowatts per square meter (kW/m2). Irradiance is measured through an instrument called ‘pyranometer,’ which displays the instantaneous power available from the Sun.

Solar constant is the solar irradiance outside the earth’s atmosphere on a 1 square meter surface oriented normal to the sun’s rays. It is about 1367 W/m2. This is attenuated by the atmosphere and the peak solar insolation on a earth’s surface oriented normal to the sun on a clear day is of the order of 1000 W/m2.

This irradiance of 1000 W/m2 corresponds to Standard Testing Conditions (STC) and is called “peak sun” or “1 sun”. If the incident radiation is concentrated 10 times using a lens or a mirror assembly and the incident power increases to 10,000 W/m2, then the irradiance is called “10 Suns.”

 Air mass is a measure of the distance traveled by sunlight through the Earth’s atmosphere. Since light intensity is attenuated by scattering and absorption, the more distance it passes through the atmosphere, the greater is the attenuation. Consequently, the sun appears less bright at the horizon (morning and late afternoon) than when at the zenith (noon). An air mass of 1 means the sun is looking straight down on the sea surface when it is directly overhead. At any location with latitude greater than 23.5 degrees, the sun is never directly overhead and so air mass will be always greater than 1. The number 1.5 has been agreed upon for the STC (Standard Test Condition) for testing solar panels.

Dear Dr. Hussein A Kaze

Thanks for this question,

I'm currently studying the performance of evacuated tube solar heater system, the solar radiation at angle 45 face south in Iraq in a range 600 to 1300 w/m2, so I think the title angle effect.

Thanks Adel. That is what I mean. Some time the solar radiation is over 1000 W/m2 on the ground, as you mentioned 1300 W/m2 in Iraq. How it come and what is the scientific reasons? beside the PV manufactured and tested in the lab to work with less than 1000 W/m2, so what is the situation when the solar radiation is over the STC solar radiation (1000 W/m2)?.

Dear Dr.Hussein A Kazem

Theoretical limit  for solar irradiance is 1 solar constant ( the energy from the sun per unit time received on a unit area of surface perpendicular to the direction of propagation of the radiation at mean earth-sun distance outside the atmosphere) The World Radiation Center (WRC) has adopted a value of 1367 W/m2 for solar constant , with an uncertainty of the order of 1%. So  any value below 1367 W/m2 is acceptable if  your pyranometer is calibrated well. For  maximum  power  input to the system  for entire year, keep PV panel facing south   at tilt angle equal to longitude of location. Pyranometer at the same tilt angle will show higher magnitude than keeping in horizontal.

these are the causes for increase in the solar irradiation

1.Total solar irradiance (TSI) changes slowly on decadal and longer timescales. The variation during solar cycle 21 was about 0.1% (peak-to-peak). In contrast to older reconstructions, most recent TSI reconstructions point to an increase of only about 0.05% to 0.1% between the Maunder Minimum and the present.

2.Ultraviolet irradiance (EUV) varies by approximately 1.5 percent from solar maxima to minima, for 200 to 300 nm wavelengths. However, a proxy study estimated that UV has increased by 3.0% since the Maunder Minimum

3.Some variations in insonation are not due to solar changes but rather due to the Earth moving between its perigee and apogee, or changes in the latitudinal distribution of radiation. These orbital changes or Milankovitch cycles have caused radiance variations of as much as 25% (locally; global average changes are much smaller) over long periods.

while the PV are manufactured according to the standard test conditions (STC is an industry-wide standard to indicate the performance of PV modules and specifies a cell temperature of 25°C and an irradiance of 1000 W/m2 with an air mass 1.5 (AM1.5) spectrum. These correspond to the irradiance and spectrum of sunlight incident on a clear day upon a sun-facing 37°-tilted surface with the sun at an angle of 41.81° above the horizon.) but this have a terms and conditions *

a set of test conditions were developed to measure solar panel performance under “real world” conditions. The conditions were called “Photovoltaics for Utility Scale Applications Test Conditions” or PVUSA Test Conditions; more commonly “PTC.”

The primary difference between STC and PTC is temperature. If you recall, under STC conditions, everything is at 77°F (25°C).

Under PTC, everything is heated up as if it were in the sun. The solar cells within the panel are raised to their “normal operating cell temperature” which is typically around 113°F (45°C). The ambient temperature is set to 68°F (20°C), and a 2.2 mph (~1 meter/second) breeze blows across the panel. 

Because of the physics of solar panels, as they heat up, the voltage they can produce goes down. And since the electrical Power (watts) = Voltage x Current, when the voltage goes down, the power goes down. 

Imagine you purchased a 7,800-watt system. If your panels could operate at STC – everything at 77°F all the time – your system might actually run at 7,800 watts of capacity at some point in time. But in the real world, the panels will run much hotter (even on a cold day), and the power will drop significantly. The PTC analysis tells us that even under modest heating, the system’s output will be about 8-10% below the STC rating. If we take the specific case of the Canadian Solar panel, with PTC of 239.1, your 7,800-watt array can, at most, run at about 7,173 watts.

but manufacturers cant design and manufacture PV 's that would be so intelligent to adjust them self for different conditions, for all these reasons they set some limit globally(STC). in fact it is like fooling all of us with something that is globally accepted unknowingly 

Thanks Bashara and Battula for your inputs. I agree with Basharat that the cloud act as concentrator and reflector, which boost the solar radiation. I know that this theorem mentioned early in eighty decade. However, the solar radiation reach more than 1400 W/m2! How? What is the effect? is it good or bad? how can we utilize it?

Depletion of ozone is one of the reason, because ozone is the prime component which absorbs uv radiation (from about 200 nm to 315 nm wavelength). in the absence of ozone this UV radiation increases the total radiation by 3%-12%, the actual STC is 1000w/m2 considering worst conditions when it is clear sky and ozone is also absent the radiation would be above 1367w/m2.

we are so lucky with our earth it is a giant generator which produces an electromagnetic field which shields us from ionizing radiations,infra red radiation, microwaves and radio waves. coming to the effects of it, if this shield is absent ....you can watch  a movie called "THE CORE" which resembles the situation i.e.,absence of electromagnetic shield.it is a perfect example you would clearly understand the effects.

it is absolutely bad condition even you know the reason why.

we can utilize that energy using some modified plasma reactors, but what if it is a point of survival and existence of life on earth  

The real phenomenon behind this (frequent) situation is called cloud enhancement. In regions of high albedo, this can be even more important. For a complete analysis, see

Article Cloud and albedo enhancement impacts on solar irradiance usi...

Article Cloud and albedo enhancement impacts on solar irradiance usi...

It has been shown in the literature that the total solar irradiance at the ground level can be enhanced up to 20 % due to refection on clouds edges (in broken cloud conditions) and / or by important albedo (e.g.in snow conditions). Albedo being wavelength dependent, the spectral distribution is slightly modified in these last conditions.