How to know voltage and current in parallel combination?

23/1/2017

We learned about the wiring of the solar panels in the series combination. It explained that in the series combination, the nominal voltage adds up while the current in the system remains the same. Now, we will see the effect on voltage and current when panels are connected in parallel.

Actually, arranging the solar panels in different combinations is the basics of designing the solar PV system. The voltage & the current output of the silicon solar cells (solar panels) vary with the change in their type of combination (series, parallel or combination of both). I am going to discuss and analyze the effect of the output voltage and the current when the solar panels are connected in the parallel combination. Remember that you can only increase the system voltage, system current or both by connecting solar panels in different configurations (series, parallel or combination of both) but in any case, you are not creating any kind of the extra power or energy. The power of the system will remain the same irrespective of the panel combination.

(1) How to connect the solar panels?

Any solar panel has two terminals one is positive and the other is negative, when the positive and the negative terminals of first solar panel are connected with the positive and the negative terminals of the second panel respectively, again the positive and the negative terminals of the second panel are connected with the positive and the negative terminals of the third panel respectively and so on, the configuration is called as parallel configuration. Please see the diagram below:

(2) The current will add up

The current in the system will add up when you connect solar panels in parallel to each other
Let’s say, I have 3 solar panels, each one of them is having the same electrical parameters or ratings:
Panel 1

Voltage (V1) = 12 volts
Current (I1) = 15 amperes

Panel 2

Nominal voltage (V2) = 12 volts
Current (I2) = 15 amperes

Panel 3

Nominal voltage (V3) = 12 volts
Current (I3) = 15 amperes

Now, when I connect these 3 solar panels in parallel, the system current will add up:

Isys = I1 + I2 + I3 = 15 amperes + 15 amperes + 15 amperes = 45 amperes

You can see the effect in the following diagram:
You can also write in a general way that when n panels with each one of them having same current I, are connected in parallel, the overall system current will become n x I

(3) The voltage remains the same

When all the solar panels connected in the system having the same nominal voltage then the system voltage will be Vsys = V1 = V2 = V3. For example, when 3 solar panels with the same nominal voltage of 12 volts are connected under parallel combination, the system voltage will be 12 volts.

(4) Understanding the overall effect

Let us understand the overall effect on the voltage and the current of the system with the help of the following example:
I have same 3 panels with the same electrical parameters & characteristics:
Panel 1

Voltage (V1) = 12 volts
Current (I1) = 15 amperes
Power (P1) = 12 volts x 15 amperes = 180 watts

Panel 2

Nominal voltage (V2) = 12 volts
Current (I2) = 15 amperes
Power (P2) = 12 volts x 15 amperes = 180 watts

Panel 3

Nominal voltage (V3) = 12 volts
Current (I3) = 15 amperes
Power (P3) = 12 volts x 15 amperes = 180 watts

When connected in parallel, the system voltage will be

Vsys = V1 = V2 = V3 = 12 volts

The system current will add up and increase

Isys = (15 + 15 + 15) amperes = 45 amperes

The power of the system, Psys = 12 volts x 45 amperes = 540 watts which are similar to the sum of the individual power of each panel that is 180 watts + 180 watts + 180 watts = 540 watts

(5) Connecting solar panels of different voltages and current

When you connect solar panels in parallel having the different electrical ratings, the current will add up to constitute the system current whereas the voltage of the system will the voltage of the solar panel having the lowest value. Let’s understand this with the help of the following example:
I have 3 panels with different voltage and the current ratings:
Panel 1

Nominal voltage (V1) = 12 volts
Current (I1) = 10 ampere
Power (P1) = 12 volts x 10 amps = 120 watts

Panel 2

Nominal voltage (V2) = 9 volts
Current (I2) = 5 amps
Power (P2) = 9 volts x 5 amps = 45 watts

Panel 3

Nominal voltage (V3) = 5 volts
Current (I3) = 2 amps
Power (P3) = 5 volts x 2 amps = 10 watts

Now, when I join these three panels in the parallel combination my system voltage and the current will be as follows:

Vsys = Vlowest = 5 volts
Isys = I1 + I2 + I3 = 10 amps + 5 amps + 2 amps = 17 amps

panels with different voltage and current

Here, you can see that my system is operating at a voltage of 5 volts as its performance is attenuated by the lowest voltage panel which resulted in the reduced efficiency of the whole system.
The output power of your solar PV system, in this case, it will be 5 volts x 17 amps = 85 watts whereas the sum of the individual power of all the three panels is 120 watts + 45 watts + 10 watts = 175 watts.
The system is operating at an efficiency of 48.57 % of its maximum power.

Here, I can say that connecting solar panels in parallel with different nominal voltages results in reduced efficiency because the solar panel with the lowest nominal voltage determines the system voltage. Therefore, the parallel configured solar system to operate at the full efficiency, each solar panel must have same nominal voltage although it is not necessary that each solar panel should have a same current rating.

Conclusion

When you are connecting solar panels in parallel, the nominal voltage of each solar panel should be same otherwise the system voltage will reduce to the extent to that nominal voltage value which is the lowest in the array of the solar panels thus reducing the efficiency of the system. When dissimilar nominal voltage panels are connected in a parallel combination, the high wattage solar panels operate at lower efficiency, results in the wastage of the money which you have invested in purchasing high power solar panel.
In simple words, when connecting in parallel, the nominal voltage of the solar panels must be the same. However, they may have different current ratings. The right configuration will increase the efficiency of your solar roof and this will not only enable you to save money in the different months of the year but also enhance the feasibility of your residential solar roof.