When wiring multiple solar panels together in a system, you have two choices: series and parallel. Determining whether you wire your solar panels in series or parallel mainly depends on your application. Let’s examine the differences and when each method is best.
This section will go into more depth on series, parallel and series-parallel connections. The purpose of this section is to explain why certain connections are utilized, how to set up to your desired connection, as well as going over what is the most beneficial connection to utilize based on your situation.
What is the difference?
The main difference between wiring solar panels in series or parallel is the output voltage and current.
When you wire multiple panels in series, their output voltages add together, and their output current remains the same. Conversely, when you wire numerous solar panels in parallel, their output currents add together, but their output voltages stay the same.
Parallel
Strictly parallel connections are mostly utilized in smaller, more basic systems, and usually with PWM Controllers, although they are exceptions. Connecting your panels in parallel will increase the amps and keep the voltage the same. This is often used in 12V systems with multiple panels as wiring 12V panels in parallel allows you to keep your charging capabilities 12V.
The downside to parallel systems is that high amperage is difficult to travel long distances without using very thick wires. Systems as high as 1000 Watts might end up outputting over 50 amps which is very difficult to transfer, especially in the systems were your panels are more than 10 feet from your controller, in which case you would have to go to 4 AWG or thicker which can be expensive in long run. Also, paralleling systems require extra equipment such as branch connectors or combiner box.
Parallel Disadvantages
Solar panels in parallel operate independently of one another and therefore are the best option for mixed-light conditions. If shade covers one or two of your panels, the remaining panels in the array will continue to generate power as expected.
Series
Strictly series connections are mostly utilized in smaller systems with a MPPT Controller. Connecting your panels in series will increase the voltage level and keep the amperage the same.
The reason why series connections are utilized with MPPT controllers is that MPPT Controllers actually are able to accept a higher voltage input, and still be able to charge your 12V or more batteries. Renogy and Victron MPPT Controllers can accept much larger voltages and amps than the stock standard Aussie brands. The benefit of series is that it is easy to transfer over long distances.
Series Disadvantages
The downside to series systems is shading problems. When panels are wired in series, they all in a sense depend on each other. If one panel is shaded it will affect the whole string. This will not happen in a parallel connection.
Solar panels in series are optimal in unshaded conditions. If shade covers a single panel of your series array, it will bring down the whole system’s power output. Each panel in a series connection is critical.
So which option is better?
Connecting your solar panels in series is often preferred if you’re able to spend most of your time in unshaded locations. The main reason for this is that your solar panel system will be more efficient and will perform better at the beginning and end of the day and when it’s cloudy. Lithium batteries are very thirsty batteries and require a good strong voltage to charge.
Batteries require a higher voltage than their nominal voltage to charge. For example, most lithium batteries need 14.4 volts to start charging. Most solar panels in the 100-watt range have an output voltage between 18-20 volts. To reach the 14.4 volts required to charge your batteries, solar panels in parallel would need to be operating at 75% capacity or more.
if you were to wire three of these same panels in series, the maximum output voltage would be 54-60 volts. This would mean your panel array would only need to operate at around 25% capacity to provide a charge to your batteries. Operating at 25% capacity is much easier to achieve even late in the day or on cloudy days. It is important to remember that you will need an MPPT charge controller for a system like this to work.
Wiring your solar panels in parallel is not necessarily a bad option. MPPT charge controllers can be more expensive and may be overkill for small portable applications.
If you have a small low-voltage system for an RV or boat that might be in variable lighting conditions, a parallel system may be a great option. Systems like this will work great with a lower-cost PWM charge controller, which will save you money. However, you will limit your future options with that charge controller choice.
Mixing Solar Panel Types
So far we have assumed that we are wiring solar panels in series or parallel using identical panels. While this is ideal and is the best option, it is not a requirement. Mixing solar panels from different manufacturers with different electrical ratings is possible, but there are some crucial guidelines to follow.
If you are wiring a set of panels in Series then you need them to have the same current rating.
Going back to the initial example if one of the 18-volt panels has a current rating of four amps instead of six amps, the output of the whole system would be four amps. This means the total power would drop from about 324 watts (54 volts X 6 amps = 324 watts) to 216 watts (54 volts X 4 amps = 216 watts).
Alternatively, when wiring your solar array in parallel, all of the panels should have the same voltage rating. The system output voltage will match the panel with the lowest voltage rating.
In general, it’s best to use the same panels in each array on a charge controller. To optimize mixing solar panel types using multiple charge controllers with each panel array on its controller will maximize solar output.
How to set up a solar system in parallel?
A Parallel connection is accomplished by joining the positives of two panels together, as well as the negatives of each panel together. This can be accomplished by different means, but usually for smaller systems this will be utilized via branch connector. The branch connector has a Y shape, and one has two inputs for positive, which changes to one, along with two inputs for negative, which changes for one. Please see picture below.
As you can see you have a slot for the negative terminal of panel #1 and the negative terminal of panel #2. As well as the positive equivalents. Then the negative out and the positive out will be utilized to connect to your charge controller via a solar PV cable.
Please see diagram below.
Let’s look at a numerical example.
Say you have 2 x 100 Watt solar panels and a 12V battery bank. Since each panel is 12V and the battery bank you want to charge is 12V, then you need to parallel your system to keep the voltage the same. The operating voltage is 18.9V and the operating current is 5.29 amps. Paralleling the system would keep the voltage the same and increase the amps by the number of panels paralleled. In this case you have 5.29 Amps x 2 = 10.58 Amps. Voltage stays at 18.9 Volts. To check math you can do 10.58 amps x 18.9 volts = 199.96 Watts, or pretty much 200 Watts.
How To Set Up Your System In Series
A Series connection is accomplished by joining the positive of one panel to the negative of the other panel together. With this you do not need any additional equipment except for the panel leads provided. Please see diagram below.
Let’s look at a numerical example. Say you have 2 x 100 Watt solar panels and a 24V battery bank. Since each panel is 12V and the battery bank you want to charge is 24V, then you need to series your system to increase the voltage. For safety, use the open circuit voltage to calculate series connections, in this case the 100 Watt panel has 22.5 Volts open circuit, and 5.29 amps. Connection in series would be 22.5 volts x 2 = 45 volts. Amps would stay at 5.29. The reason we use open circuit voltage is we have to account for the maximum input voltage of the charge controller.
*If you want to check math it won’t work with the open circuit voltage. You can use the operating voltage, so 18.9 volts x 2 = 37.8 volts. 37.8 volts x 5.29 amps = 199.96 Watts, or pretty much 200 Watts.
How to set up a Series Parallel System?
A series-parallel connection is accomplished by using both a series and a parallel connection. Every time you group panels together in series, whether is 2, 4, 10, 100, etc. this is called a string. When doing a series-parallel connection, you are essentially paralleling 2 or more equal strings together.
As you can see this series parallel connection has 2 strings of 4 panels. The strings are paralleled together.
The voltage and current are much higher in this arrangement. Most likely this setup is for an off grid house rather than a caravan or RV.
In this example you would use the open circuit voltage of 22.5 Volts and the operating current of 5.29 Amps. Creating a string of 4 panels, you will have a voltage of 22.5 Volts x 4 = 90 volts, which is under the 100 Volt limit. Then by paralleling on the other string, the voltage will stay 90 volts and the amps will double, so 5.29 amps x 2 = 10.58 Amps.
* Keep in mind there is usually another factor that needs to be taken into account when sizing for the MPPT Controller called the boost current. This will be discussed in the charge controller section.