Are solar panels wired in series or parallel?

Solar panels can be wired in series or parallel and in some cases, it might be a combination of both. The operating current and operating DC voltage of the inverter or charge controller decides the maximum number of solar panels that can be connected in series or parallel. Arranging solar panels in series or parallel is called string design and this is an integral part of the design works.

This article deep dives into the key design parameters of solar panels and inverters that helps to decide if solar panels must be connected in series or parallel.  

How does the back of a solar panel look?

If you look at the flipside of a solar panel, you will see a black box and two cables coming out of this box. The black box is called a junction box (IP 68 rated) and there is one positive and one negative cable coming from this box. The junction box is IP 65 rated. The cables are single core copper with a cross section of 4mm2. You will find MC4 connectors used at the terminals of the cables.

How you connect these cables determines if it’s a series or parallel connection. 

What is a series connection?

A series connection is formed by connecting the positive terminal of one panel to the negative terminal of the next panel. A series connection can also be achieved by connecting the negative terminal of one panel to the positive terminal of the next panel, as shown in the figure below. 

What is a parallel connection?

In a parallel connection all positive terminals are connected together, and all negative terminals are connected together. In the figure below it can be observed all positive cables are taken to a common point and from this point one single positive cable goes to the charge controller. Similarly, all negative cables are taken to a common point and from this point one single negative cable goes to the charge controller. This is called a parallel connection. 

What happens if you increase solar panels in series?

When solar panels are connected in series it is called a string. You can have multiple strings in a solar installation.

When solar panels are connected in series, the voltage adds up while the current remains the same. For example, in a 400 Wp (Wp stands for watt peak - maximum power the solar panel can produce) solar panel, the maximum power current is 13.15 A and the open circuit voltage is 36.98 VDC. When you connect 400 Wp solar panels in series, the voltage multiples by the number of solar panels you connect in series while the current remains constant. For example, if you connect 10 solar panels in series, the voltage will be 370 VDC while the current remains at 13.15 A. 

What happens if you increase solar panels in parallel?

When solar panels are connected in parallel, the voltage remains constant while the current increases. For example, in a 400 Wp solar panel, the maximum power current is 13.15 A and open circuit voltage is 36.98 VDC. When the solar panels are connected in parallel, the voltage remains constant while the current multiples by the number of solar panels connected in parallel. For example, if you connect 10 solar panels in parallel, the voltage remains at 36.98 VDC while the current increases to 131 A.

What decides the maximum number of panels you can connect in series and parallel?

The short answer is the inverter or charge controller decides the maximum number of solar panels you can connect in series or parallel.

Let’s consider two examples to understand this better. In example 1 we will consider a 5 kWp solar installation and in example 2 we will consider a 10 kWp solar installation.

Example 1: 5 kWp rooftop solar installation. For this example, let us consider a standard 300 Wp solar panel and GoodWe GW5k-DT inverter.

A standard 300 Wp panel has an open circuit voltage of 44.71 VDC and a short circuit current of 8.94 A.

GW5k-DT inverter can accommodate a maximum of 7.5 kWp of solar panels and has a max. DC input voltage of 1,000 VDC. The maximum input current is 12.5 A.

The first step is to understand roughly how many solar panels you will need for a 5 kWp installation. Each solar panel considered is 300 Wp, you will need approx. 16 solar panels for a 5 kWp installation.  

There are two ways one can execute this project. You can have 2 strings consisting of 16 panels or 1 string consisting of 16 panels. Let’s look at both options.

Option 1: 8 solar panels are connected in series to form the first string and another 8 solar panels are connected in series to form the second string. In each of the strings the string voltage is 357 VDC while the string current is 8.94 A. The GoodWe inverter supports 2 MPPT`s and 1 string per MPPT. Each of the stings are connected to one input terminal of the inverter. 

Figure 4: 5 kWp string layout option 1

Option 2: 16 panels are connected in series, increasing the string voltage to 715 VDC while the current remains constant at 8.94 A. The string is then directly connected to one MPPT input of the inverter. 

Figure 5: 5 kWp string layout option 2

Example 2: 10 kWp roof top solar installation. For this example, let us consider a standard 300 Wp solar panel and GoodWe GW10kT-DT inverter.

A standard 300 Wp panel has an open circuit voltage of 44.71 VDC and a short circuit current of 8.94 A.

GW10kT-DT inverter can accommodate a maximum of 15 kWp of solar panels and has a max. DC input voltage of 1,000 VDC. The maximum input current is 12.5 A.

The first step is to understand roughly how many solar panels you will need for a 10 kWp installation. Each solar panel considered is 300 Wp, you will need approx. 34 solar panels for a 10 kWp installation. 

Unfortunately, if we connect 34 panels in series, the string voltage will be over 1,000 VDCa limiting factor on the inverter. So, in this case we only have one option, which is to split the 34 panels into 2 strings of 17 panels in each string.

String 1 will have 17 panels, string voltage for 17 panels connected in series is 760 VDC and the current is 12.5 A. Similarly string 2 will have 17 panels.

So how do you decide between series and parallel?

The first step is to confirm the solar capacity you want to install. Second step is to understand how many solar panels you need. Third step is to find a suitable inverter and understand the specifications on the inverter. Fourth and final step is to design the string configuration based on solar panel and inverter specifications.