How to measure the efficiency of your solar roof?

12/6/2015

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 so as to design an optimum solar power system. If you are having space constraints then you need more efficient solar panels and if you have ample space for your solar roof then you can trade off with less efficient solar panels.

Power under Standard Test Conditions

(More efficient solar panels produce more wattage per unit area and require less space while less efficient solar panels need more space to produce the same amount of wattage)
Let us calculate the efficiency with the help of an example:
The following table shows the Panel maximum power under STC and the area of the panel.

S.no	Watts (Pm)	Area (mÂ²)
1	200	1.475
2	250	1.643
3	300	1.941

It is the area (length x breadth) of the solar panel which decides its efficiency.

Efficiency of the solar panel

The radiation flux is the amount of sunlight falling on the surface of the earth. It is measured in $Watt / m^2$. The radiation flux under Standard Test Conditions is $1000 W/m^2$
Now, calculating the efficiency of the solar modules of different Pmax and the area:

S.no	Watts	Area (mÂ²)	Efficiency
1	200	1.475	13.56%
2	250	1.643	15.22%
3	300	1.941	15.46%

$È max$ (Maximum efficiency 200 watt panel) = (200 / 1000) x (1.475) x 100 = 13.56%
$È max$ (Maximum efficiency 250 watt panel) = (250 / 1000) x (1.643) x 100 = 15.22%
$È max$ (Maximum efficiency 300 watt panel) = (300 / 1000) x (1.941) x 100 = 15.46%

Now, you can see that per unit efficiency of the 300 watt panel is the highest.
Let us suppose that you want to install 3 KW roof top solar system to meet your daily energy needs, but you have a limited space at the top which is suitable for the installation. The total shadow free area available is $23 m^2$, now you have to find out the best possible system which can be accommodated at your roof top and at the same time is the most efficient.

Area required

Let us calculate the number of solar panels required and the total area covered by them.

S.no	Watts	Area (mÂ²)	Efficiency	No of Panels required for 3 KW system (mÂ²)	Total area covered by them (mÂ²)
1	200	1.475	13.56%	15	22.125
2	250	1.643	15.22%	12	19.716
3	300	1.941	15.46%	10	19.41

For 200 watts solar panels,

Number of panels required = 3000/200 = 15 panels
Area covered by them = 15 x 1.475 m2 = 22.125 m2

Similarly for 250 watts panels,

Number of panels required = 3000/250 = 12 panels
Area covered = 12 x 1.643 = 19.716 m2

For 300 watt panels,

Number of panels required = 3000/300 = 10 panels
Area covered = 1.941 x 10 = 19.41 m2

Now, you can see that the area covered by the 300 watt solar panels is the smallest and at the same time per unit efficiency is the highest that is 15.46%.
(I am not taking into consideration the gap in between the two panels which is generally provided for air to pass through to the keep the panels cool)
Therefore, 300 watt 10 solar panels having 15.46% efficiency are ideal to meet your desired demand and can be accommodated in the limited space at the roof top.

If you already have installed the solar roof at your house, then also you can calculate the efficiency of your solar roof as follows:
$È max$ (Maximum efficiency of the solar roof) is as follows:

$È max$ (Maximum efficiency of the solar roof) = Pmax (Maximum power output of one panel) x total number of panels installed / Ef (radiation flux) x Area of one Panel x total number of panels installed x 100

(Assuming that all solar panels at the roof are identical in size)
So whenever next time you decide to buy a panel, consider its efficiency before paying the price for it. You might have ample roof area which can accommodate less efficient panels. But when you are having a limited space, then efficiency plays a major role in designing the solar power system which can meet your electricity demand in a limited roof top area.

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