The feasibility of the solar power system mainly depends on the amount of sunlight your panels are receiving.
Your panels are performing well if you live in the tropical region of the earth as it receives a good amount of sunshine. You probably see less of sunlight when you move away from a tropical region, affecting the profitability of the solar system.
Moreover, our earth revolves around the sun and is simultaneously spinning about its axis is the reason we receive a different amount of sunlight falling on the panel each hour of the day. We see different seasons due to this phenomenon of earth.
The indirect method
The one method could be to measure the electrical output by connecting the electrical meter and divide that value by the efficiency of the solar panel. The efficiency is always mentioned at the back of it.
You get the amount of sunlight falling on the panel at that moment. But this is an indirect method of measuring it, implying more chances of error in the measurement.
The direct method
The pyranometers measure the amount of sunlight falling on the panel more accurately and in a scientific way.
The solar pyranometer is an instrument which measures these solar radiations per unit area (W/m²) on the earth’s surface within a range of 300 nm to 3000 nm
The electromagnetic radiations in the form of energy falling per unit area on the flat surface is called the solar irradiance.
You may see sunlight as yellow, orange, red or white but it consists of a broad spectrum of electromagnetic radiations from 280 nm to 4000nm, including visible white light consisting of VIBGYOR and invisible electromagnetic radiations like ultraviolet and infrared radiations.
Broadly speaking, there are 2 types of pyranometers one is thermopile type and another is solar cell type pyranometer.
Although both do the same thing, one measures the sunlight from the heat it generates while another measures the sunlight from the electricity it generates.
When sunlight falls on the pyranometer, the black carbon element absorbs most of the light which thermopile placed under it, notices and responds by generating an equivalent amount of current.
Let me tell you that a thermopile is an instrument which produces current due to the temperature difference between 2 elements to which it is connected.
The output current is maximum when the sun is overhead in the noon and it is minimum at dawn or dusk. The output current is a function of cosine θ, where θ is the angle between sun rays and the vertical line to the surface of the pyranometer.
Solar cell type Pyranometer
Not all Pyranometers are thermopile based, there are pyranometers which measures the sunlight using the light-sensitive solar cells.
These pyranometers are not as accurate as thermopile types as they only work and respond in a limited band say 300 nm to 1100 nm of the electromagnetic spectrum.
If you are not making accurate measurements then I think these are fine for your needs as they are cheaper too.
Construction and Working of solar cell type pyranometer:
Probably, you get an idea that using pyranometers we can accurately measure the sunlight falling on the panels.
However, you can roughly assess the performance of your solar system without them by observing the following things: