How to know voltage and current in the series?

You are sitting in your room and planning to fulfill your energy needs through solar power.

After finding the solar roof feasibility, you decide to size the solar panels by calculating the daily electricity demand of your house and come to know that it is, say 3 KW per day.

But there is no single panel in the ​market which can generate 3 KW alone in a day. The maximum wattage of the solar panels used or available in the residential sector is 300 watts.

It means one single solar panel can produce up to the extent of 300 watts of power. Therefore, you need to connect a number of solar panels in a certain pattern so as to produce the desired output power. ​

Well, whenever you add solar panels, it affects the voltage and current of the overall system which depends on the type of combination (series, parallel or both) you are choosing while connecting the solar panels.

The solar panels must be connected in the certain pattern so as to produce the required output voltage and current that matches the electrical ratings of other devices (like battery, inverter and the solar charge controller) which help in their right sizing.

These patterns can be any one of the following:

• The solar panels are connected in series
• The parallel combination
• The combination of both series and parallel

In this article, I am going to discuss the about the series combination of the solar panels in the system.

As you all know that any solar panel has 2 connecting terminals one is called the positive terminal while the other is the negative terminal.

In series combination, the negative terminal of one panel is connected to the positive terminal of the second panel and the negative terminal of the second panel is connected to the positive terminal of the third panel and so on.

​ It can be shown in the diagram as follows:

Whenever you configure your solar panels in any of the combinations, there are changes in system voltage, system current or both. Now, in the case of the series combination, the following change takes place:

(1) The voltage of  the solar panels add up in the series combination

When you connect your solar panels in the series combination, the voltage of the each solar panel gets to add up.

For example, you have three solar panels of 300 watts each and each one is having a voltage of 12 volts when connected in series, their voltages will add up (12 volts + 12 volts + 12 volts) to give system voltage of 36 volts.

​You can write in a general way that when n panels are connected in series with each panel having voltage V, then the system voltage will become n x V.

(2) The current in the system remains the same

Let us understand this with the help of the following example, I have 3 solar panels of 300 watts each with each of them is having following voltage & current rating

• Voltage = 12 volts
• Current = 25 amperes
• Power = 12 volts x 25 amperes = 300 watts

When I connect these panels in the series combination, the following changes will take place,

• System voltage = 12 + 12 + 12 = 36 volts
• System current = 25 amperes

Total power of the system = 36 volts x 25 amperes = 900 watts (which is same when you add the individual power of each panel, 300 watts + 300 watts + 300 watts = 900 watts)

(3) It is not recommended to connect panels the different configuration

It is important that two panels should have same electrical parameters. When two panels having different current ratings, are connected in series, they tend to have different performance degradation which can affect overall system performance.

Therefore, the solar panels of similar current rating should be connected in series combination. For example, when you connect solar panel of 25-ampere rating with the solar panel of 20 amperes rating, the system current will drag down to 20 amperes, hence lowering the overall power of the system and negatively affecting its performance.

Let us further understand this with the help of the following example:
The three panels are having following electrical parameters:
Panel 1 & 2

• Voltage 12 volts
• Current =25 amperes
• Power = 300 watts

Panel 3

• Voltage 12 volts
• Current = 20 amperes
• Power = 240 watts

When these 3 panels are connected in series, the system voltage becomes 12 volts + 12 volts + 12 volts = 36 volts and the system current becomes 20 amperes (panel having the lowest current rating).

Therefore, the overall power of the system will become 36 volts x 20 amperes = 720 watts (which is less the sum of the individual power of the 3 panels, 300 watts + 300 watts + 240 watts = 840 watts).
​
The above series combination of solar panel of different current rating results in loss of power of (840 watts – 720 watts) 120 watts.

Conclusion

In any of the combinations, you are not creating any kind of extra power, the total power of the system will remain the same, in the above case, it is 900 watts.

We can only minimize the losses by choosing and connecting the solar panels of similar electrical parameters.

Remember, when panels are connected in series the voltage adds up and the current is same only when the each panel is having the same current rating.

If any of the panels connected in the series is having the lower current rating in comparison to the other panels, it will lower down the current flowing in the other panels and will impact their individual performance. 

Their under performance will affect the efficiency of solar roof that will produce lower current and power than mentioned in the nameplate.

In simple words, when two different panels are connected in series, the voltage still adds up but the system current is equal to the current of the panel having the lowest current rating.

​It is you who has to decide what should be the system voltage and the system current of your solar PV system so that it can match up with your other devices like battery, inverter and the solar charge controller.

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• How much money your solar rooftop is saving in the different months of the year?