How to know solar panel efficiency in 4 steps?

Solar panel efficiency, let us first understand the word “efficiency”.

I use the word “efficiency” casually in my daily routine. 

It is the ratio of the amount of useful work performed by anybody to the total input given.

“Technically speaking, it is the ratio of useful work performed by any machine to the total energy given as an input.” For example, a steam engine has an efficiency of 60%.

​It means that it is able to convert 60% of the steam energy into mechanical energy.

The rest 40% is lost to the surroundings.

Efficiency = Useful work-done/ Total energy given

Similarly, the efficiency of your solar roof is its ability to convert sunlight into usable electrical energy for consumption.

It is important to know the efficiency of the solar panels for designing an optimum solar power system.

​If you are having space constraints then you need more efficient solar panels (efficient solar panels have less cross-sectional area than less efficient ones with the same power rating).

And

if there is ample shade-free space on the roof then you can trade-off with less efficient solar panels.

Electrical ratings of solar panels under STC

The technical specifications like power ratings, voltage ratings, and current ratings of your solar panel are the values obtained under Standard Test Conditions.

What is standard about them?

The sun-light of value 1000 watts/m² (called solar irradiance) is allowed to fall on the panel surface. The other important condition is cell temperature of the solar panel is kept at 25° C or 77° F when the light source is falling on its surface and with no wind flowing around it.

The manufacturers then measure the voltage and current values and label them on the back of the panel. 

Knowing the maximum power of the solar panel

The maximum power (Pmax) is measured indirectly which is the product of voltage and current;

Max.Power = Max. Voltage x Max. Current

Or

P(max) = V(max) X I(max)

​Thereafter, the power rating is also marked behind the panel. 

“​More efficient solar panel requires less space while less efficient one needs more space to produce the same amount of power.”

The manufacturers make panels with ratings like 100 watts, 200 watts, 250 watts, and 300 watts so that it is easy to design the configuration like 1 kW, 2 kW, 3 kW, and so on.

The solar panels are generally rectangular in shape with small thickness when compared to their length and breadth.

Its dimensions usually come under the category Mechanical ratings as:
Length x Breadth x Height

For measuring the efficiency of any solar panel we take its cross-sectional area that Length x Breadth, ignoring the Height.

Because it is the area of the panel that receives the sunlight.

Please see the screenshot of a piece of the datasheet of 400 W (Pmax) solar panel of Solaria Power:

You can see that its dimensions are:
1644 mm x 1204 mm x 40 mm
(Length   x  Breadth   x Height)

Ignoring the Height, we’re left with Length x Breadth.

Calculating its area: 1.644 m x 1.204 m = 1.98 m²

The value of incident light (1000 watts/m²), power rating of the panel, and the area occupied by it become the basis for calculating the efficiency of the solar roof.
​
*(The area of the panel is calculated by multiplying its length and breadth)

How to know the solar panel efficiency?

Although the efficiency of the solar panel is printed at the back of it.

The manufacturers follow certain steps to calculate it and that you too can do it.

Let’s start it:

Step1: Sunlight

Know the sunlight intensity under STC and it is 1000 W/m²

Step 2: Pmax

Write the Pmax of the solar panel whose efficiency you want to calculate.

Step 3: Solar panel dimensions

Write the dimensions (Length x Breadth) of the panel and calculate its cross-sectional area.

Step 4: The efficiency

Calculate its efficiency by putting all values of the above steps in the following equation:

That’s it you got the answer!

An example for finding the solar panel efficiency

Let us calculate the efficiency of the Solaria Power solar panel from the piece of information given below:

• Sunlight under STC = 1000 W/m²
• ​P(max) = 400 Watts
• ​Area = 1.644 m x 1.204 m = 1.98 m²

Putting these values in the formula:

We calculated it as 20.2%

Now let me reveal the efficiency of the Solaria Power 400 W solar panel from the datasheet.

Here we go
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That GREAT!

we did it accurately, it is the same as what we calculated now.

Similarly, you can easily know the efficiency of any panel. 

“If you have two panels with the same power ratings, the one which is occupying the less area is efficient.”

But the efficient panels come at a higher price. 

Comparing solar panel efficiency

You can check out by looking at the datasheets of 2 different panels with the same power rating:

The efficiency of 2 solar panels with the same power rating is inversely proportional to their cross-sectional area.

Conclusion

If you have ample roof space then buying the less efficient solar panels can be a prudent approach as these are cheaper in price.

But when you are having limited space then efficient solar panels can be a real help in designing the solar power system capable of meeting your electricity demand in a limited area.

Are you interested in installing a solar system on your roof-top?

Then my solar feasibility spreadsheet could be a great help in determining the complete technical and financial feasibility beforehand.