How to connect solar panels

We receive many questions such as ” how to connect solar panels in series or parallel to get the maximum power” , or ” how to connect solar panels to obtain the best performance” . The answer is simple:  There is only one way to connect the plates properly solar!  And of course the power is the same regardless of whether they are connected in series, in parallel or in series parallel.

Too Many Solar Panels and Few Batteries

The power does not change depending on the connection of the solar panels.

Power is governed by the formula  P = V x I ; where the power is constant. Therefore if we increase the voltage (V) the current (I) will decrease and vice versa, but the power will be the same. Let’s see a real example:

12V and 150W solar panel with the following characteristics:

Power at the point of max. power  (Pmax) 150Wp
Open circuit voltage  (Voc) 22.6V
Voltage at maximum power  (Vmpp) 18.2V
Short-circuit  current (Isc) 8.7A
Current at maximum power  (Impp) 8.25A

We can verify that at maximum operating power: Vmpp: 18.2V x Impp: 8.25A = 150.15Wp

If we have 2 solar panels of 150Wp we have a total of 150wp x 2 =  300Wp  and as we will see below, the power does not vary regardless of how we connect them.

If we connect them in series:  (we add the voltages and the current is maintained)

• Vmpp: 18.2V x 2 = 36.4V
• Impp: 8.25
• Power total: Vmpp: 36,4V x Impp: 8,25 =  300,3Wp

Impp: 8.25A x 2 = 16.5A If we connect them in parallel:  (we add the currents and the voltage is maintained)

• Vmpp: 18.2V
• Power total: Vmpp: 18,2V x Impp: 16.5 =  300,3Wp

How to connect solar panels well

With PWM regulator

If the battery of the solar installation is 12V:  The solar panels must be 12V and very important 36 solar cells. The only way to connect 12V solar panels with 12V batteries is by connecting the solar panels in  parallel . In this way, the set of solar panels will work with a voltage as close as possible to the maximum power point of this type of solar panels. PWM regulators are  simply in charge of controlling the charging voltages of the batteries. They are not capable of modifying the working voltage of the solar panels, therefore the solar panels will always work at the battery voltage.

If the battery is 24V:  We can put groups of 2 12V solar panels and 36 cells connected in series to form 24V panels and 72 solar cells, or use 24V solar panels and 72 solar cells. Again, each group of panels in series or 24V solar panels and 72 solar cells can only be connected in parallel to work at the voltage closest to the maximum power point of this type of solar panel.

Never exceed the maximum load current of the PWM regulator.

Do not use 60-cell solar panels with PWM regulators.

With MPPT regulator

There are 5 factors that we must take into account to know how to connect the solar panels to the MPPT regulators: The MPPT regulators  are capable of separating the working voltage of the solar panels or group of solar panels from the battery voltage. This feature makes it possible for the solar panels to always work at their maximum power point regardless of the voltage at which the battery is.

1.- Maximum charge current.  The charge current is the current that will go from the MPPT regulator to the battery. It is NOT the current that comes out of the group of solar panels. To calculate it we will divide the power of the photovoltaic generator by the battery voltage.

For example: 3 solar panels of 250Wp are 750Wp, for a 24V battery the charging current will be: 750Wp / 24V = 31.25A. In this case, a 30A or higher MPPT solar regulator can be used. In case we use a lower MPPT, it will limit the charging current to the maximum supported by the solar controller.

If the battery were 12V we would have: 750Wp / 12V = 62.5A. In this case a 60 amp MPPT solar regulator would be required. Therefore, the higher the battery voltage the better because we will need smaller regulators.

2.- Maximum voltage Vmax. MPPT regulator.  The most used are Vmax: 100V and Vmax: 150V

• With Vmax: 100V regulators we can connect a maximum of 2 solar panels of 60 cells in series.
• With Vmax: 150V regulators we can connect a maximum of 3 solar panels of 60 cells in series.

If you use 36-cell or 72-cell solar panels.  Be very careful with the open circuit voltages Voc of the plates. If the Vmax value of the MPPT solar controller is exceeded, it will be damaged. The MPPT solar regulators were designed to be used with 60 cell solar panels , therefore with these panels they are very easy to configure. On the other hand, with solar panels of 36 or 72 cells it is more difficult to square the connection so as not to exceed the Vmax of the regulator.

3.- The ideal is to work at the maximum possible voltage in the photovoltaic field.

The higher the voltage of the photovoltaic field, the lower the current (do not confuse the current of the photovoltaic field with the battery charging current, since in MPPT regulators it is not the same). The advantage is that the cable section will be smaller and we will have less losses in the cables due to heating. In addition, we can place the solar panels at greater distances from the solar regulator.

Thus, if the photovoltaic generator works at 60V the current will be half that if it works at 30V. If the solar regulator allows it, it will be better to work at 90V.

4.- The solar panels of the same chain must be the same. And the strings must be the same.

If a chain is made up of 2 solar panels in series, the other chains must be of the same number of solar panels. Solar panels in series add voltages and strings in parallel add current to the photovoltaic field.

Therefore we can only form sets like this:

• 2 solar panels: 2 panels connected in series (Vmpp: 60V)
• 3 solar panels: 3 panels connected in series (Vmpp: 90V)
• 4 solar panels: 2 parallel strings of 2 plates in series (Vmpp: 60V)
• 6 solar panels: 2 parallel strings of 3 solar panels in series (Vmpp: 90V)
• 6 solar panels: 3 parallel strings of 2 solar panels in series (use the previous configuration if the controller allows it) (Vmpp: 60V)
• 8 solar panels: 4 parallel strings of 2 solar panels in series (Normally you cannot put more than 3 solar panels in series) (Vmpp: 60V)

Note:  inverters with MPPT are capable of working with 200V to 600V made up of sets of 9 to 20 solar panels in series. MPPT solar regulators for battery installations normally allow 100V or 150V maximum.

5.- The voltage of the photovoltaic field must be at least 2.5V higher than that of the battery at all times.

We will not be able to configure a photovoltaic field that works at 30V with a 24V battery, since the battery must reach 28.8V to charge 100% and the photovoltaic field will have an insufficient working voltage for perfect operation.

CONCLUSION

Before buying the solar panels you need to know if the solar kit will have an MPPT or a PWM solar controller, if the battery voltage will be 12V, 24V or 48V.