Mixing solar panels – Dos and Don’ts • SOLAR POWER SECRETS (2024)

Connecting more than one solar panel in series, in parallel or in a mixed-mode is an effective and easy way not only to build a cost-effective solar panel system but also helps us add more solar panels in the future to meet our increasing daily needs for electricity.

How to connect your solar panels depends on:

• The type of your solar panels system,
• The solar power you want to generate,
• The other system components, such as a charge controller, battery, and inverter.

There are two main types of connecting solar panels – in series or in parallel. You connect solar panels in series when you want to get a higher voltage.

If you, however, need to get higher current, you should connect your panels in parallel.

Should you need both a higher voltage and a higher current, you have to apply both connection modes, which means that a part of your solar panels should be wired in series, while the remaining ones are to be wired in parallel.

The most important to remember is that both connection modes provide you with a higher wattage.

Therefore, if the power output of a solar panel cannot alone meet your daily electricity needs, you should think of adding more such panels to it, whether in series or in parallel.

To get the maximum efficient solar panel system, however, you should keep some basic principles related to connecting solar panels.

Last but not least don’t forget about the overcurrent protection of the solar panels and the solar power system!

Wiring solar photovoltaic panels in series

As we said above, when connecting solar panels in series, we get an increased wattage in combination with a higher voltage.

Such ‘higher voltage’ means that series connection is more often applied in grid-tied solar systems where:

1) the system voltage is often at least 24 volts, and

2) the solar array output voltage is fed to an inverter or charge controller of a typically higher input voltage compared to off-grid systems.

Connecting in series means joining the positive terminal of a solar panel to the negative terminal of the next solar panel until eventually you are left with one free positive and one free negative terminal of the array, which are to be connected to the input either of the inverter (in case of a grid-tied system without a battery backup) or the charge controller (in case of a grid-tied system with a battery backup or off-grid solar panel system).

When you connect solar panels in series, the total output current of the solar array is the same as the current passing through a single panel, while the total output voltage is a sum of the voltage drops on each solar panel.

The latter is only valid provided that the panels connected are of the same type and power rating.

Let’s consider the depicted below solar panels designated for a 12V solar panel system, operating at their Maximum Power Point, while delivering the depicted voltage and current that correspond to this power tracking point.

Wiring solar panels of different ratings in series

Here is a series connection of solar panels of different voltage ratings and the same current rating:

You can see that if one of the solar panels has a lower voltage rating (and the same current rating) compared to the remaining panels, the output power is lower than in the previous example but the loss is not significant. Things, however, are entirely different if you connect in series panels of different current ratings.

You should, however, have in mind that the current produced from а solar panel depends on the ambient temperature, solar cells temperature, and solar irradiance.

If the lower wattage solar panel is from different series or a different brand, it might behave differently under the same ambient conditions.

For example, if under the same environmental conditions the solar panel of the different wattage (i.e., 136W) has a lower current (for example, 7.5A), it would drag the performance of the whole solar array down, because it would limit the solar array’s current to 7.5A.

The performance of the solar array is as strong as the performance of the weakest element.

In a series connection, such a weak element is the solar panel with the lowest current.

The following example reveals this in more details.

This time we have a series connection of solar panels of different voltage ratings and different current ratings:

In this picture, you can see that a total of three different types of solar panels are used.

Each panel type has its own voltage, current, and power rating.

The total current here is determined by the panel of the lowest current rating and, as a result, the total wattage is severely reduced (by 40%) compared to the previous example where the loss of output power is not so significant.

Furthermore, if you take a look in the first panel in the row, and assume that you have wired four such panels in parallel, then the total output power would be: 4 x 85W = 340W.

Just compare this to the dramatically reduced wattage of 365W, and you’ll find out that if you connect in series solar panels with different voltage and current ratings, the total output power is determined mostly by the solar panel of the lowest rating!

What is more, let’s imagine an ideal fictitious situation where the current does not influence the performance of the solar array – the total harvested solar power would be 515W (85W+126W+152W+152W)!

Wiring solar pv panels in parallel

The next basic type of connecting solar panels is in parallel.

Connecting solar panels in parallel is just the opposite of series connection and is used to increase the total output current of the array, and hence the total output power while keeping the same voltage.

‘The same voltage’ is the system voltage which for off-grid solar panels systems is usually as low as either 6V or 12V.

For this reason, parallel connection is more typical for off-grid systems.

In the parallel connection, all the positive terminals of the panels are joined together, and all the negative terminals are also joined together.

Eventually, you have one common positive and one common negative terminal of the solar array which are to be connected to input either of the inverter (in case of a grid-tied system without a battery backup) or the charge controller (in case of a grid-tied system with a battery backup or off-grid system).

When you connect solar panels in parallel, the total output voltage of the solar array is the same as the voltage of a single panel, while the total output current is a sum of the currents passing through each panel.

The latter is only valid provided that the panels connected are of the same type and power rating.

Wiring solar panels of different ratings in parallel

Here is a parallel connection of solar panels of different voltage ratings and the same current rating:

As you can see, things are getting worse, since the total voltage of the array is determined by the solar panel of the lowest voltage rating: we received 11% loss of installed solar power.

Let’s see what happens when we bring even more diversity and connect in parallel solar panels of different voltage and current ratings:

Things are steadily getting worse, but it’s evident that what you lose here as wattage is much lower compared to connecting different solar panels in series.

Important to summarize:

Both in series and parallel connection, plugging a panel of a lower power rating to the array drags the whole output power down.

The lower the rating, the higher the loss of solar generated power.

This, however, is much more crucial for panels connected in parallel.

Therefore, if you want to get the maximum power from your solar array, you should only connect similar panels.

Mixing different panels, whether connected in series or in parallel, ALWAYS reduces the installed wattage.

Furthermore, if you don’t have any other option than wiring dissimilar panels, you should know that:

1) For series connection – the same current rating of the panels is more important.

2) For parallel connection – the same voltage rating of the panels is more important.

Mixed wiring of solar panels

A combination of series and parallel connection is also possible.

Indeed, this depends on the maximum possible total output voltage and maximum possible total output current of the solar array, which are limited by the maximum input voltage and the maximum input current of your charge controller (for off-grid systems) or inverter (for grid-tied systems).

Calculations show you how many panels and how to wire.

What you have to remember is that the series connection increases voltage, while the parallel connection increases current.

Both series and parallel connection result in more installed wattage, which goes down should you decide to wire solar panels of different ratings.

If you decide to apply a mixed connection, it’s practical your solar array to comprise an even number of panels (a multiple of 2), for example, 4 panels (2 in series and 2 in parallel) or 6 panels (3 in series and 2 in parallel).

If the system sizing calculations result in an odd number of panels (for example, 3 or 5), and you are sure you are not going to add more panels in the future, it’s practical to use either series or parallel wiring.

If anyway you prefer wiring solar panels of different ratings rather than spending money to buy similar solar panels and end up with an installed wattage you’ll never need, it is a smart idea to separate the panels in two sets and wire them in parallel.

In such a case, however, you should either search for a charge controller (or inverter, for grid-tied systems) with at least two input feeds or install a second charge controller (or inverter, for grid-tied systems).

As you see, with a solar array comprising different solar panels, the only way to avoid losses of the installed wattage is separating the panels in individual circuits, for the sake, however, of possibly more complicated wiring and more expensive charge controller or inverter.

What we recommend:
1) Use panels that have the same ratings.

2) When connecting different solar panels, in order to minimize the losses:

• Connect only in series panels of the different brands and of the same current.
• Connect in parallel panels of different brands and of the same voltage.
• Connecting different solar panels in a solar array is not recommended since either the voltage or the current might get reduced. This leads to lower output power, and hence to less solar-generated electricity. Therefore, if you are planning to use dissimilar panels, try to pick ones with similar voltage and current.
• Every different panel in a solar array has a different optimal solar power tracking point at a given moment. Let’s suppose that you are using an MPTT charge controller. Different solar panels reduce the effectiveness of the controller to track this optimal power point. An MPPT solar charge controller is a smarter device than a PWM charge controller regarding its capability to squeeze more solar power by tracking the optimal power point of the PV panels or solar array. While searching for the optimal compromise, however, it is not smart enough to triangulate between so much different optimal power points.

How to squeeze more solar power from different solar panels by breaking the best practice rules?

You might say, I have these different panels.

I’ve bought them. It was a real bargain.

I don’t want to throw them away.

What can I do to squeeze the optimal solar power from them?

Yes, you can do that, but it requires some investment.

You should buy additional charge controller(s) and try to isolate these dissimilarities by implementing ‘Ideal Mixing.’

Here is an Ideal Mixing of different solar panels of the same voltage, connected in parallel, with the help of a charge controller:

Scenario 2. The solar panels are of voltage rating higher than the system voltage.

You have two different higher voltage solar panels, i.e., one 100W/24V and one 200W/24V that you want to connect to the already working 12 V solar power system comprising the two 12V 50 W solar panels connected in parallel from the previous scenario(see the picture above).

In this case, you have to use a step-down MPPT charge controller capable of stepping the 24 V solar panel voltage down to 12V.

Why MPPT charge controllers?

Why not using the low-cost PW controller you might ask?

Because the MPPT charge controllers convert the voltage difference between 24V solar panel and 12V battery bank to an increase in its output current that is twice higher compared to using a PWM charge controller.

With this twice higher current, the power provided at the output of the MPPT controller would be almost two times higher compared to the usage of a step-down PWM controller.

Please, have in mind that this a simplified diagram. Hence, the equipment needed for combining these solar panels and fuses needare omitted.

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