April 16, 2018 Solar Energy Written by Greentumble Editorial Team
Connection of solar panels
To design a solar PV system

for any household, it is necessary to consider several parameters like the available solar resource, amount of power to be supplied by the system, solar panel efficiency, autonomy of the system (off-grid or connected to the grid) as well as the selection of components like inverters, batteries and controllers.

Beyond the analysis of these components, there is another element that can deeply influence the total power output of the system and can also affect the overall cost of the solar installation. It is the connection of solar panels.

We often hear the installers talking about the connection of solar panels in series or parallel, but many of us not related to the technical terms do not understand the difference between these designs, and therefore, do not understand the impact our decision has on the overall lifecycle of the system.

Here we will give you a brief explanation of the general design process and the reasons for the selection of one or another type of connection.

The first thing that you must know is that in any power system the variable that matters the most is the active power (expressed in Watts). Why? Because all electrical devices consume the active power to function.

Moreover, the equation that determines this variable is P=V*I, where V is related to voltage and I is related to current. As you can see, these are the only variables that allow to modify the power output of a solar system, keep this in mind as it will be important later.

To obtain the desired active power, there are three ways of connecting your solar panels together to create a power system that provides solar electricity to your house; they are defined as:

    • connection in parallel

    • connection in series

    • a combination of the two (series-parallels).

The decision of one or another connection will depend on the desired output and application of the system as we will discuss in this article
 

Connecting solar panels in parallel

Parallel connection implies wiring positive terminals of each panel together and wiring the negative terminals of each panel together as well. Then, they are connected to the charge controller or to the inverter of the system. When solar panels are connected in parallel (known as arrays) they all share the same voltage, and the current that each one of them provides is summed up.

The main advantage of this configuration is related to the concept of reliability, as in the case that any of the solar panels is affected either by shade or by any damage in manufacturing process or along the lifecycle of the system, then, the performance of the other solar panels is not affected because the wiring connection makes every single unit independent from the other one.

On the other hand, there are some disadvantages of this connection. Low voltages denote higher current values, which translates into higher electrical losses (power losses are related to the square values of current), and therefore, lower efficiency performance of your PV system.

Besides, increasing the current is not desirable either because it implies an increase of wire gauges in order to have better capacities for enduring higher ampere values (associated with higher temperatures and therefore security issues), which at the end translates into higher installation costs due to the bigger size of the cables and also due to an increase in the length of the cables to be able to perform the connection [1].
 

When should the installation be done in parallel connection?

The answer is relative, but in most cases, we assume that the system is very small and is meant to supply low loads, or has a battery set to a low voltage design (12 Volts, for example).
 

Connecting solar panels in series

The series connection is done by wiring the positive terminal of each panel to the negative terminal of the next panel (a connection similar to the ones of the Christmas lights) until the final panel is connected to the charge controller or inverter.

In the series connection the voltages of all solar panels are summed up and the current is maintained the same for all the items. The set of panels connected in series is known as a string.

As stated before: lower voltages imply higher currents and higher voltages imply lower currents.

This statement is a very important for series connection, because as this configuration increases voltage values with every added panel, then, the overall current provided by the system will be lower.

This translates in savings due to lower wire sizes and cable length, and also in higher efficiency of the PV system (lower electrical losses).

However, the main disadvantage of this configuration is low reliability of the system when connected in series. In other words, as the whole system is connected by a single cable, if the cable fails then all the system is totally affected.

When one of the solar panels in the string is shaded, whether by a tree or a cloud, the overall performance and efficiency of the system is affected. Shading can even become a bigger problem in this configuration because a shaded part creates resistance in the current flow. This effect creates so called hotspots. Hotspots are sections with increased temperature, and depending on the wiring and the system size can even represent a security problem.
 

When should the series connection be used?

Generally, when higher voltages are desired, the logical solution is the series connection. Besides, when the distance between the inverter or charge controller and the solar panels is long (20 feet or more), it is advisable to use a series connection because the voltage drop in the process can be elevated enough as to fall below the requirement of the inverter’s voltage input and as the series connection allows to increase the voltage of the system, this provides the installer some maneuverability.

The series connection has its issues as stated before, but the shading and efficiency problem can be solved by using one of these two considerations:

    • The first one is related to the efficiency. Selecting a micro-inverter for each panel provides the best solution to achieve maximum efficiency, as every panel will be independent from the other one [2]. Therefore, shading of one panel will not affect the overall power output.

    • The second option is to buy a solar panel with a bypass diode. This diode acts as a wall that blocks or isolates the shaded area of the panel in order to avoid efficiency losses of the rest of the same panel and therefore of the entire power system [3].

Unfortunately, the reliability issue cannot be solved, as it is an intrinsic property of the connection itself.
 

Series-parallel connections

So, we have seen the advantages and disadvantages of the two main available connections, but what happens if we combine them?

It is often necessary to establish the connection according to the voltage and the current input range of the inverter or the charge controller. But in some cases, it is not possible to achieve the required range of voltage and current through only series or parallel connections. In case of series connection, voltage limit could be exceeded, and in the parallel connection, current limit could be exceeded while at the same time not supplying the maximum possible active power of the configuration [4]. That is when a combination of the two connection is needed.

The idea is to establish strings (series connection of two or more panels) and connect them in parallel with other strings (creating arrays of strings). This allows to obtain the advantages of the series connection (lower electrical losses and lower costs) and the benefits of the parallel connection (reliability).

In other words, each string of panels will add a current and will also be independent from the performance of another string, while at the same time obtaining higher active power with higher voltage and lower current values (desirable due to security measures).

In this configuration creativity is the rule of thumb, as with the same number of panels, multiple design options can be created that adjust to the purposes of the homeowner or to the requirements of the power components.
 

Combining solar panels with different electrical characteristics

There is another important topic related to the selection of one or another type of connection in the solar PV system.

Do your solar panels share the same electrical characteristics?

So far we have assumed that your system contains a set of very similar or equal solar panels, meaning the same manufacturer, same electrical efficiency and characteristics (voltage, current and active power). But what happens if you decide to connect different panels?

Remember the intrinsic characteristics of each type of connection, the parallel connection forces all the system to have the same voltage and the series connection forces all the system to have the same current.


Consider having a set of 4 panels, 3 panels of 12V/3A and 1 panel of 9V/1A.

If you connect these four panels in parallel, all of them must have the same voltage, and therefore, will generate at the maximum possible voltage for one of the panels, which means 9V.

Ptot = P1 + P2 + P3 +P4 = 9V * (3A + 3A + 3A + 1A) = 90 W.

When you take a look at the expression P=V*I, you realize that the input of each panel will be dropped in voltage (instead of the output voltage 12V, you obtain only 9V output for each one). This phenomena influences directly the overall efficiency of your PV system, because you are underutilizing the total capacity of the panels and therefore increasing the marginal cost of your system.

Now, suppose that you connect the same panels in series, the same problem is presented. You will sum up the voltages but the current will be the lower one.

Ptot = P1 + P2 + P3 + P4 = (12V + 12V + 12V + 9V) * (1A) = 45 W.

As you can see, the obtained power output is very different from one configuration to the other, and if solar panels had been selected equally, then, the total active power would had been Ptot= 144 W for any of the two configurations.


Therefore, it is not advisable to combine solar panels with different specifications, because low performance and underutilization will affect the system.

Even selecting different manufacturers with the same electrical characteristics is not advisable because besides the rated power, each panel has a power degradation percentage which is never the same among different manufacturing processes. In the long term, you will have some panels degrading at different rate than other. This presents instability and could also lead to higher electrical losses.

As you can see, the selection of one or another connection type is supported by the deep technical analysis and the range of possibilities can be very wide if the system is large enough.

But always remember that the major decision makers to establish the type of connection that your system should have are:

    • Specifications of inverter’s or charge controller’s power input

    • Size of your system

    • Distance between inverter and panels

    • Expected active power output


 


References

[1] https://goo.gl/Abiu8G
[2] https://understandsolar.com/solar-panels-in-series-vs-parallel/
[3] http://www.mpptsolar.com/en/solar-panels-in-series.html
[4] https://www.renogy.com/learn-series-and-parallel/