How to Measure Solar Irradiance on Your Panels (With Real Example & Formula)

If your solar panels are underperforming, the first thing you should check is not the inverter — it’s the sunlight input. Solar irradiance is the real “fuel” of your solar power system.

Just like machines need quality input to work efficiently, your solar panels rely on sunlight – technically known as solar irradiance. In simple words, it is the total amount of sunlight (both direct and diffused) falling on the solar panels.

Read: Understanding Solar Irradiance: DNI, DHI, and GHI Made Simple

It matters a lot for the solar panels.
Why?

Because it directly affects the amount of electricity your solar panels are producing. And without enough sunlight input, your solar panels can never reach their full potential. Whether you are sizing a new solar power system or checking the performance of the existing one, knowing its value is important.

Measuring the value of solar irradiance at any point in time in a day, often called instantaneous solar irradiance, can tell us the amount of DC power your panels are producing. Over time, its daily or monthly value helps us in evaluating the financial feasibility of going solar in that location.

What is Solar Irradiance?

It is the power of the sunlight received in 1m² area. And it is measured in Watts/m².

There are two types:

• Direct solar irradiance: The sunlight coming directly from the sun
• Diffused solar irradiance: The sunlight reaching the Earth’s surface after scattering from the clouds, dust particles, water vapours, etc.

And the sum of direct and diffuse solar irradiance is the Global Horizontal Irradiance, which matters a lot for solar panels.

On a horizontal surface, Direct + Diffuse = Global Horizontal Irradiance (GHI).
On a tilted solar panel, we measure Plane-of-Array (POA) irradiance.

Why do we measure Solar Irradiance?

Measuring the solar irradiance helps us in the initial pre-checking of the solar panels’ output by:

Identifying the shading issues: If your solar panels are generating less power despite a good value of solar irradiance, then this could be due to the shading effect.

Understand whether your location is good for solar or not: A good value of solar irradiance in a region generally indicates that solar is attractive. Like in India, the yearly average is 5-5.5 kW/m²/day, making it an attractive destination for solar.

Calculate solar output and financial returns: The solar electricity produced is directly proportional to the value of solar irradiance. The money saved per month from going solar depends on the units generated in that month. This helps us in calculating the payback period of solar.

Check if the system is working properly or not: If your solar panels are not producing the desired output despite a good value of solar irradiance and being shade-free. Then it indicates some technical issue with the solar panels. It could be a diode damage or cracking in the solar cell.

Now, let us look at the different methods of measuring the solar irradiance.

Method 1: Measuring Solar Irradiance Using Pyranometer

It is a device that measures the amount of sunlight falling on a surface. It tells you how much solar energy is available at a given moment of time- just like a thermometer tells you the temperature.

Working

• The protective lens protects the inner structure and allows the sunlight to enter it from all directions above the surface.
• The sunlight reaches the black surface (perfect absorber). The black surface absorbs the sunlight and gets heated.
• The thermopile placed below the black surface takes in the heat and converts it into equivalent voltage.
• This voltage is collected across the terminals, and the GHI value is measured in W/m².

The more intense the sunlight, the higher the irradiance reading.

It is usually placed on a horizontal surface, but some models may be placed on tilted solar panels to measure radiation.

On a sunny day at noon, the Pyranometer may show up to 900-1000 W/m².

Method 2: Using output power and solar panel specifications

This is the easy method to know the solar irradiance value if you have solar panels installed on the rooftop.

By using the simple formula:

G = P/(Area x n)

Where;

• G = Solar irradiance being measured in W/m²
• P = Real-time power output from the solar panels in Watts
• A = area of the solar panel in m²
• n = number of solar panels in the system
• η = Efficiency of the solar panel (in decimal)

Instantaneous vs Daily Solar Irradiance – What’s the Difference?

When we measure solar irradiance using the formula above, we are calculating instantaneous irradiance. This is the real-time sunlight intensity at a specific moment — measured in W/m².

For example: At 12:10 PM, your panels may be receiving 750 W/m². This helps us:

• Check real-time system performance
• Detect shading or faults
• Verify whether panels are underperforming

But what about daily irradiance?

Daily irradiance is measured in kWh/m²/day. Instead of a single moment, it represents the total sunlight received over the entire day. This is what we use for:

• Annual generation estimation
• Solar system sizing
• Payback period calculation
• Financial feasibility studies

For example: If your city receives 5.5 kWh/m²/day annually, solar economics will be much stronger compared to a region receiving only 3.5 kWh/m²/day.

• Instantaneous irradiance tells you how your system is performing right now.
• Daily irradiance tells you whether solar is financially attractive in your location.

Want to Calculate Your Exact Solar Payback?

Measuring irradiance is just the first step.

To know:

• Your exact monthly generation
• Export losses
• Real savings (not brochure numbers)
• Accurate payback period
• 25-year ROI

Use my Solar Feasibility Spreadsheet (SFS) — an engineering-based tool built for homeowners. 👉 Before investing lakhs in solar, calculate everything in minutes. Get the Solar Feasibility Spreadsheet – India Edition and US Edition

Let us understand the whole with the example of my solar panels’ output:

Real Example: Measuring Irradiance Using My 5 kW Solar System

Before applying the formula, we must ensure that the system conditions are ideal. Otherwise, the calculated irradiance will not reflect actual sunlight intensity.

Steps to Follow:

(i) Are solar panels clean?
Yes

(ii) Is wiring in good conditrion?
Yes

(iii) Is there any shade over solar panels?
No

(iv) Note the time of the day
Time: 12:10 pm (August 7, 2025)

(v) Measure the solar output from the inverter
Output Power = 4010 Watts

(vi) Measure the area of the solar panel
Area = l x b = 2.280m x 1.135m = 2.5878 m²

(vii) Count the number of solar panels
The number of solar panels is 10, each of 530 Watts, total power 5.3 kW

(viii) Know the efficiency of the solar panels
Each solar panel is a monocrystalline, half-cut solar cell technology with an efficiency of 20.49% or 0.2049

Putting all these values in the formula:
G = P/(Area x n x η)
G = 4010/(2.5878 x 10 x 0.2049)G = 756.26 W/m²

Final Thoughts

Solar irradiance is not just a scientific term — it is the foundation of your solar plant’s performance and profitability. When you understand:

• How much sunlight your panels actually receive
• How to calculate irradiance using real output
• And how it affects long-term generation

You stop depending on installer assumptions and start making data-driven decisions. Because in solar, sunlight is not just energy — it is economics. And measuring it correctly is the first step toward intelligent solar investing.