How & Why to Wire Solar Panels in Parallel or in Series

In this blog, we will go over the basics of wiring solar panels in parallel and in series. Then, using my off-grid system as an example, we’ll see why we would use one or the other, or both. Let’s start off with a quick comparison of parallel and series circuits.

Note: Just like batteries, solar panels have a negative terminal ( – ) and a positive terminal ( + ). Current flows from the negative terminal through the load (current consumed by a piece of equipment) to the positive terminal.

Warning!!!

All content of this blog is for educational purposes only. Use at your own risk. Do-It-Yourself (DIY) projects involving electricity, construction and associated tools can damage equipment, void property insurance & product warranties, be dangerous and even fatal to yourself and others. Proper safety procedures should be followed at all times. YOU take full responsibility for your actions. I take NO responsibility for any results of your actions or lack thereof. Do your own due diligence!

This may also be a good place to add, DO NOT CONNECT OR DISCONNECT DC CIRCUITS UNDER LOAD! DC current arcs more readily than AC current and can even weld the connections. DC circuits should only be opened or closed by switches or breakers designed specifically for that function.

Parallel Circuits

Circuits wired in parallel have multiple paths for the current to move along. If an item in the circuit is broken, current will continue to move along the other paths, while ignoring the broken one. This type of circuit is used for most household electrical wiring. For example, when you turn off your TV, your lights still remain on.

To wire solar panels in parallel, simply connect all the positive terminals of all the solar panels together, and all the negative terminals of all the panels together (see below). When wiring solar panels in parallel, the amperage (current) is additive, but the voltage remains the same. For example, if you have 4 solar panels in parallel and each is rated at 24 volts and 10 amps, the entire array would be 24 volts and 40 amps. The total watts (24 volts x 40 amps) will be 960 watts.

Series Circuits

Circuits wired in series have only one path for current to travel along. Therefore, all the current in the circuit must flow through all the loads. A series circuit is a continuous, closed loop – breaking the circuit at any point stops the entire series from operating. An example of a series circuit is a string of old Christmas lights – if one bulb breaks, the whole string turns off.

To wire solar panels in series, simply connect the positive terminal of the first solar panel to the negative terminal of the next one, and so on (see below). When wiring solar panels in a series, the voltage is additive, but the amperage remains the same. For example, if you have 4 solar panels in a series and each is rated at 24 volts and 10 amps, the output of the entire string would be 96 volts and 10 amps.

As you can see, whether we wire the panels in series or parallel, both configurations produce the same amount of power: 960 watts. Only the volts and amps differ.

Series or Parallel?

There are a number of factors that need to be taken into account when determining whether to wire in series or parallel, or a combination of the two.

• Battery nominal voltage
• Panel maximum output voltage
• Panel maximum output amperage
• Charge controller minimum/maximum input voltage
• Charge controller minimum/maximum input and output amperage

There are other factors that come into play, but these are the primary variables we need to be concerned with at this point. Many configurations will not change by considering the other variables, but some may be better optimized. We will take a look at additional considerations to fully optimize a configuration later in this series.

Example of Applying Series & Parallel

The best way to understand the application of these variables is to apply them to a real-world situation. So let’s take a look at my own off-grid system as it is. Then we will cover why it is as it is. As I mentioned in Going Off-Grid – Overview of My Home System, what looks like one large array of 24 solar panels, is actually two separate arrays.

So a high-level diagram looks something like this:

As I mentioned in Going Off-Grid – Overview of My Home System, each of the arrays is made up of four strings. A string is simply a group of panels that are connected in series. Each string contains three panels. Now let’s see each string in the arrays:

Wiring a String in Series Example

As you can see in the above diagram, each array is identical. Each contains a red, blue, green and black color-coded string of three panels wired in series. Now, let’s take a look at a wiring diagram of each string:

As mentioned earlier, to wire solar panels in series, we simply connect the positive terminal of the first solar panel to the negative terminal of the next one, and so on. Remember, when we wire in series, the voltage is additive, but the amperage remains constant. So 3 panels x 44.9 volts gives a total maximum output voltage of 134.7. And the maximum amperage for the entire string remains at 8.73.

Combining the Strings in Parallel Example

Just as my solar array looks like a single array, but is actually two separate arrays (West & East), my combiner box contains two distinct combiners (West & East). Since the two combiners are mirror images of each other, let’s remove the smoked plastic cover and zoom in on the right (East) combiner for more detail.

Now that we can see things more clearly, you will notice that all of the positive (+) wires from the strings (E1, E2, E3 & ES) in the East array, each connect to a 15 amp DC circuit breaker. Then all four of those breakers connect to a common bus bar. Similarly, all of the negative (-) wires from the strings (E1, E2, E3 & ES) connect to a common bus bar (the white diagonal bar). As you will recall, to wire solar panels in parallel, we simply connect all the positive terminals of all the solar panels together, and all the negative terminals of all the panels together. But in this case, we have connected all the positive terminals of all the strings together, and all the negative terminals of all the strings together. So, the strings are each wired in series, but then combined in parallel.

Then notice that both bus bars, negative and positive, each have a larger 6 gauge wire connected to them (both marked PV2 East). These two wires take the combined power of the entire East array to one of the two Outback FLEXmax 80’s. So, now we have a wiring diagram that looks like this:

Total Power of East Array

Recalling that when we combine circuits in parallel, the amperage is additive, but the voltage is constant – by combining (in parallel) all four strings (in series) from the East array, we now have a total maximum output of 134.7 volts and 34.92 amps.

At this point, some of you may have a question: “If the FLEXmax 80 charge controller can handle 80 amps, why not just have all eight strings from both arrays in one array? After all, 34.92 x 2 is only 69.84 amps, well below the 80 amp maximum rating of the controller.” That’s a very good question.

Above is a picture of the display on one of the charge controllers. If you look at the first line of the display, this is the “Input” line, or what is being produced and coming from the array. Since it is early in the day, the voltage is just a hair over 100 @ 16.0 amps.

The second line is the “Output” line, or what is being sent to the battery for charging. Since I have a 48V battery, the charge controller reduces the 100.3 volts to the appropriate 54.5 volts to charge the battery. At the same time, the charge controller converts the excess voltage into increased current (from 16.0 amps to 28.3 amps). This is nearly double the input amps – and we are only at 100.3 volts! So, if we were to include all 24 panels into a single array, the 69.84 maximum output of all 24 panels would easily become far in excess of the 80 amp maximum on the output of the controller. This would become even more extreme on 36, 24 or 12 volt batteries.

We don’t have to use too much brain power to figure out all the conversion to ensure we don’t go over the maximum of 80 amps. Outback Power provides a few fields on the spec sheet to assist us. These will help to ensure we don’t go over the 80 amp maximum rating.

Since my system uses a 48V battery, the specs show that the maximum array size I can use with the FM80 is 4000W. If I were to use all 24 of my panels, each rated at 305 watts, my total array power output would be 7,320 watts. However, by splitting my panels into two separate arrays of only 12 panels each, the total array output is cut in half, or 3,660 watts. This is under the 4000 watt maximum, with some room to spare.

Maximize MPPT Efficiency Through Series Size

MPPT charge controllers work at their maximum efficiency when the input voltage (that coming from the array) is as close as possible to their maximum input voltage, without going over that limit. Since my panels have an Open Circuit Voltage (VOC) of 44.9 volts, stringing the panels into series of four would give an output voltage of 179.6. This is well over the operating maximum of 145 volts shown in the FM80 spec sheet. However, three panels x 44.9 volts gives a total maximum output voltage of 134.7. This is comfortably under the 145 volt spec and gives even more wiggle room for our hot southern climate.

PWM Charge Controllers

I have mentioned in other blog posts that PWM charge controllers are not very efficient and should not play a role in even medium-sized solar systems. That being said, there is a place for smaller PWM systems. Typically, solar panels in PWM systems will be wired in parallel. Some exceptions would be stringing low voltage panels in series to match higher voltage battery systems. As with MPPT systems, just make sure to limit your maximum voltage and current to the given charge controller.

String Sizing Calculators

There are many string sizing calculators available online. Some are web based. Others are downloadable applications. Many are free. Some are not. Many are specific to the manufacturers. These can be helpful, but I would not rely upon them. All information they provide should be verified.

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