How do you calculate on grid solar system


Number of Panels required for On-grid solar system 

if I assumed that I had a solar panel manufactured by Solar World, it is a leading company in the world and I wanted to perform the calculations. I will need the following values ​​from the datasheet of the Panel :


-Cell temperature used for standard test conditions (STC), temperature coefficient of Voc
-Maximum power point voltage (Vmp)
-Temperature coefficient of Vmp
if we chose a random model of this company and we found theses values on its datasheet

Voc: 45.9, ºC @ STC: 25

TCVoc: -0.304
Vmpp: 36.7
TC Pmpp: -0.43%

Now if I take inverter  made by SMA company It is a well-known company for those working in this field, The inverter model is SMA Sunny Boy 7700TL-US-22

What interests me in the accounts from the Datasheet of the inverter is :

Maximum system voltage: Vmax( 600 Volt)

Minimum system voltage: Vmin (150 volts)

Now that I have specified the specifications of the Solar Panel and the inverter, the information I need from the datasheet should specify the weather data for the location where the installation will take place.

We record the lowest temperature at the location where the panels are to be installed and suppose which is just an assumption – 10 is the lowest temperature

We record the highest temperature and assume 38 degrees which are also just an assumption
Now we will start by calculating the number of the top or maximum panels that must be connected in series, we have the temperature at the STC conditions It is 25 ° C.

Now we cut this minimum temperature down from the Maximum  temperature of the site

38 °-13 ° =25 ° C
We multiply the temperature difference  (25 degrees) by the temperature coefficient of Voc and the Temperature coefficient 

25* TCVoc = 25*0.0034=0.107 ° C

then we multiply by Voc in STC >>> 0.017*45.9=4.88V

So 4.88V  is the voltage that will rise for each solar panel when the temperature reaches -13 and it is the lowest temperature, which is the worst possibility

Now we add this increase of 4.88 volts to the open-circuit voltage

V+Voc=4.88V + 45.9V = 50.78 

Now we add this increase of 4.88 volts to the open-circuit voltage

When we divide the maximum system voltage of the inverter, which is 600 volts, by 50.78
we get 11.8 

The maximum number of panels that can be connected within a single String is 11.8, and therefore the maximum number of solar panels is 11 in a String, If the number exceeds 11, the inverter will be separated when the temperature reaches the coldest day of the year, which is – 11degre.

As I mentioned, it is only just an example, and it can be – 1, 0 or even 5 degrees, and this depends on the region that will be done Install the system in it if we multiply 
11 x 50.78 V = 558 Maximum System voltage

We start calculating the maximum number of panels that can be connected in series, and here we will use the Vmp value, not the Voc in our calculus.

Now the highest temperature we assumed is 37 degrees Thus, the cell temperature will be approximately 25 degrees above the Perimeter of the Panel 

37+25=62  degree
 Hence the difference between cell temperature and temperature at STC conditions :
62-25=37  degree 
We multiply the temperature difference (37 degrees ) by the Vmp temperature factor, which is the same as the power coefficient of power (TC Pmpp -0.43)

TC Pmpp -0.43 %/°C

36.7 x (-0.0043) = -0.157

-0.159 x37 = -5.83V

After that the Voltage become 

-5.83 V + 36.7V=30, 87 V 
if we divided the value of Minimum system voltage on 30.87 
we will  get  4.85V
Thus, we round the number to 5 and we get a minimum of 5 Panels
then 5*30.87=154.35 V
Thus, if the temperature rises to 37 degrees as we assumed, the inverter will remain working, which is the highest temperature we assumed.

I want to comment here at the end of this article 

  1.  The first design or simulation is made to determine the maximum number of Panels based on a Voc value.
  2. The design was made  to determine the minimum number of panels based on Vmp values

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