Solar panel wiring (aka stringing), and how to string solar panels together, is a fundamental topic for any solar installer. You need to understand how different stringing configurations impact the voltage, current, and power of a solar array. This makes it possible to select an appropriate inverter for the array and make sure that the system will function effectively.The stakes are high. If the voltage of your array exceeds the inverter’s maximum, production will be limited by what the inverter can output (and depending on the extent, the inverter’s lifetime may be reduced) . If the array voltage is too low for the inverter you’ve chosen, the system will also underproduce because the inverter will not operate until its “start voltage” has been reached. This can also happen if you fail to account for how shade will affect system voltage throughout the day.
Thankfully, modern solar software can manage this complexity for you. For instance, Aurora will automatically advise you on whether your string lengths are acceptable, or even string the system for you. However, as a solar professional, it’s still important to have an understanding of the rules that guide string sizing.
In this article, we review the basic principles of stringing in systems with a string inverter and how to determine how many solar panels to have in a string. We also review different stringing options such as connecting solar panels in series and connecting solar panels in parallel.
Solar panel wiring is a complicated topic and we won’t delve into all of the details in this article, but whether you’re new to the industry and just learning the principles of solar design, or looking for a refresher, we hope this primer provides a helpful overview of some of the key concepts.
In order to understand the rules of solar panel wiring, it is necessary to understand a few key electrical terms—particularly voltage, current, and power—and how they relate to each other.
To understand these concepts, a helpful analogy is to think of electricity like water in a tank. To expand the analogy, having a higher water level is like having a higher voltage - there is more potential for something to happen (current or water flow), as illustrated below.
What Is Voltage?
Voltage, abbreviated as V and measured in volts, is defined as the difference in electrical charge between two points in a circuit. It is this difference in charge that causes electricity to flow. Voltage is a measure of potential energy, or the potential amount of energy that can be released.
In a solar array, the voltage is affected by a number of factors. First is the amount of sunlight (irradiance) on the array. As you might assume, the more irradiance on the panels, the higher the voltage will be.
Temperature also affects voltage. As the temperature increases, it reduces the amount of energy a panel produces (see our discussion of Temperature Coefficients for a more detailed discussion of this). On a cold sunny day, the voltage of a solar array may be much higher than normal, while on a very hot day, the voltage may be significantly reduced.
What Is Current?
Electric current (represented as “I” in equations) is defined as the rate at which charge is flowing. In our example above, the water flowing through the pipe out of the tank is comparable to the current in an electrical circuit. Electric current is measured in amps (short for amperes).
What is Electric Power?
Power (P) is the rate at which energy is transferred. It is equivalent to voltage times current (V*I = P) and is measured in Watts (W). In solar PV systems, an important function of the inverter—in addition to converting DC power from the solar array to AC power for use in the home and on the grid—is to maximize the power output of the array by varying the current and voltage.
For a more technical explanation of how current, voltage, and power interact within the context of a solar PV system, check out our article on Maximum Power Point Tracking (MPPT). In it, we discuss current-voltage (IV) curves (charts which show how the panel output current varies with panel output voltage), and power-voltage curves (which show how panel output power varies with panel output voltage). These curves offer insight into the voltage and current combination(s) at which power output is maximized.
To have a functional solar PV system, you need to wire the panels together to create an electrical circuit through which current will flow, and you also need to wire the panels to the inverter that will convert the DC power produced by the panels to AC power that can be used in your home and sent to the grid. In the solar industry. This is typically referred to as “stringing” and each series of panels connected together is referred to as a string.
In this article, we’ll be focusing on string inverter (as opposed to microinverters). Each string inverter has a range of voltages at which it can operate.
There are multiple ways to approach solar panel wiring. One of the key differences to understand is stringing solar panels in series versus stringing solar panels in parallel. These different stringing configurations have different effects on the electrical current and voltage in the circuit.
Connecting Solar Panels in Series
Stringing solar panels in series involves connecting each panel to the next in a line (as illustrated in the left side of the diagram above).
Just like a typical battery you may be familiar with, solar panels have positive and negative terminals. When stringing in series, the wire from the positive terminal of one solar panel is connected to the negative terminal of the next panel and so on.
When stringing panels in series, each panel additional adds to the total voltage (V) of the string but the current (I) in the string remains the same.
One drawback to stringing in series is that a shaded panel can reduce the current through the entire string. Because the current remains the same through the entire string, the current is reduced to that of the panel with the lowest current.
Connecting Solar Panels in Parallel
Stringing solar panels in parallel (shown in the right side of the diagram above) is a bit more complicated. Rather than connecting the positive terminal of one panel to the negative terminal of the next, when stringing in parallel, the positive terminals of all the panels on the string are connected to one wire and the negative terminals are all connected to another wire.
When stringing panels in parallel, each additional panel increases the current (amperage) of the circuit, however, the voltage of the circuit remains the same (equivalent to the voltage of each panel). Because of this, a benefit of stringing in series is that if one panel is heavily shaded, the rest of the panels can operate normally and the current of the entire string will not be reduced.
There are several important pieces of information about your inverter and your solar panels that you need before you can determine how to string your solar array.
Information About Your Inverter
You’ll need to understand the following inverter specifications which can be found in the manufacturer datasheet for the product:
- Maximum DC input voltage (Vinput, max) - the maximum voltage the inverter can receive
- Minimum or “Start” Voltage (Vinput, min) - the voltage level necessary for the inverter to operate
- Maximum Input Current
- How many Maximum Power Point Trackers (MPPTs) does it have?
- As noted above, a function of inverters is to maximize power output as the environmental conditions on the panels vary. They do this through Maximum Power Point Trackers (MPPTs) which identify the current and voltage at which power is maximized. However, for a given MPPT, the conditions on the panels must be relatively consistent or efficiency will be reduced (for instance, differences in shade levels or the orientation of the panels). However, if the inverter has multiple MPPTs then strings of panels with different conditions can be connected to a separate MPPT.
Information About Your Solar Panels
In addition to the above information about your selected inverter, you’ll also need the following data on your selected panels:
- Open circuit voltage (Voc)
- Short circuit current (Isc) - (although we won’t delve into current calculations in this article)
An important thing to understand about these values is that they are based on the module’s performance in what is called Standard Test Conditions (STC). STC includes an irradiance of 1000W per square meter and 25 degrees Celsius (~77 degrees F). These specific lab conditions provide consistency in testing but the real world conditions a PV system experiences may be very different.
As a result, the actual current and voltage of the panels may vary significantly from these values. You’ll need to adjust your calculations based on the expected minimum and maximum temperatures where the panels will be installed to ensure that your string lengths are appropriate for the conditions the PV system will encounter as we’ll discuss below.
When stringing your solar array, one of the basic considerations is to ensure that the voltage of the strings you are connecting to the inverter is not going to exceed the inverter’s maximum input voltage or fall below its minimum/start voltage. You’ll also need to avoid exceeding its maximum current.
You’ll also need to ensure that the maximum voltage complies with code requirements in the area where you are designing. In the U.S., the National Electrical Code caps the maximum allowable voltage at 600V for most residential systems. In Europe, higher voltages are allowed.
We know that voltage is additive in series strings while current is additive in parallel strings. As such, you might intuitively assume that you can determine the voltage of our proposed PV system design and whether it falls within the recommended range for the inverter by multiplying the voltage of the panels by the number in a series string (as illustrated in the example in the green box below).
Voltage Maximum and Minimum Calculations based on STC Values (not temperature adjusted):
Assumptions: I am using 300W panels with an open circuit voltage (Voc) of 40V. The inverter I plan on using has a maximum voltage (Vmax) of 600 V and a start voltage (Vstart) of 150 V.
I can get an initial rough understanding of the maximum number of panels that can be included on a string in series by dividing the inverter’s maximum input voltage by the Voc of the panels:
600 V / 40 V =
I can follow the same process, but using the start voltage, to determine the minimum number of panels I can include on a string.
150V / 40 V = minimum of 3.75 panels
BUT, as we discuss below, this doesn’t give the entire picture. You’ll need to adjust based on temperature.
You might also assume that you could determine the current of the system by adding the current of each parallel string (which would be equal to the current of the panels multiplied by the number in the parallel string).
However, as we discussed above, since STC values reflect the modules’ performance under very specific conditions, the actual voltage of the panels in real-world conditions may be quite different. Thus the simplified calculations above only give you an initial rough estimate; you must account for how the voltage of the system will change depending on the temperatures it may experience in the area where it is installed. At colder temperatures, the voltage of the system may be much higher; at higher temperatures, it may be much lower.
To ensure that the temperature-adjusted string voltage is within the input voltage window of the inverter, the following formulas can be used:
Aurora solar design software automatically performs these calculations and alerts you as you are designing if your string lengths are too long or too short given the expected temperatures at the site. (For more information on stringing in Aurora, see this help center article.)
Aurora also performs a variety of other validations to ensure that the system will operate as expected and not violate codes or equipment specifications—this can prevent costly performance issues. (For a detailed overview of these validations see this page in our help center.)
For a real-world example of why it is so important to accurately account for how environmental conditions will impact the voltage of your PV system, read our analysis of an underperforming system in Cathedral City, California. In that case, a solar designer’s failure to account for the presence of shade resulted in the system frequently falling below the inverter’s start voltage and therefore producing significantly less energy than forecasted.
2. Ensure Strings Have Similar Conditions—or Connect Strings with Different Conditions to Different MPPT Ports
Once you’ve determined that your strings are acceptable lengths for the inverter specifications, another key consideration is to ensure that the strings to have the same conditions (e.g. same azimuth/orientation, same tilt, same irradiance) if they are connected to the same inverter MPPT.
This is because mismatches in the conditions on the strings will reduce the efficiency and power output of your solar design (for a discussion of why mismatches in shading, orientation, or azimuth result in lost power output see the fourth article in our PV system losses series).
If you are designing for a site where it’s necessary to have panels on different roof faces, or some areas of the array will get more shade than others, you can ensure that the panels with different conditions are separated into their own strings, and then connect those strings to different MPPTs of the inverter (provided your chosen inverter has more than one MPPT). This will allow the inverter to ensure each string operates at the point where it produces the maximum power (its maximum power point).
The above rules will ensure that your stringing configuration will comply with the specifications of your inverter and that the energy production of the system won’t be negatively affected by mismatches in the conditions on the panels.
However, there are additional factors that a solar designer can consider to arrive at the optimal design (that is, the design that maximizes energy production while minimizing cost). These factors include inverter clipping, the use of module-level power electronics (MLPE)—devices which include microinverters and DC optimizers, and design efficiency provided by software tools.
Sometimes it may make sense to oversize the solar array that you are connecting to the inverter leading to a theoretical maximum voltage that is slightly higher than the inverter max. This may allow your system to produce more energy (because there are more panels) when it is below its maximum voltage, in exchange for reduced (“clipped”) production during the times when the DC voltage of the array exceeds the inverter’s maximum. If the production gains exceed the production lost to inverter clipping, then you can produce more power without paying for an additional inverter or one with a higher voltage rating.
Of course, this decision should be made with care and a clear understanding of how much production will be clipped compared to how much additional production will be gained at other times. In its system loss diagram, Aurora indicates how much energy will be lost to clipping so that you can make an informed decision about whether this makes sense. For a detailed explanation of inverter clipping and when a system with inverter clipping makes sense, see our blog article on the subject.
Module-Level Power Electronics (MLPE)
String inverters are not the only inverter option. Microinverters, which are inverters that are attached to each individual panel (or a couple), allow each panel to operate at its maximum power point regardless of the conditions on other panels. In this arrangement, one need not worry about ensuring panels on the same string have the same conditions. Microinverters can also make it easier to add more panels in the future. We discuss MLPE in more detail in this article.
Explore a Few Different Options to Find the Best One
As you can see, there are many considerations when it comes to stringing your panels and finding the inverter and stringing configuration that are best for the customer. You may not arrive at the optimal design the first time around so it can be helpful to evaluate a few different options. In order for this to be efficient, however, you’ll need a process where you can evaluate multiple designs quickly, as Aurora co-founder Christopher Hopper explains in this blog post. This is where solar software can be particularly valuable.
Let Solar Software Do the Stringing For You
Finally, new technology developments like Aurora’s autostringing functionality (discussed here) can actually do the stringing for you! It will take into account the considerations discussed here and present you with an ideal stringing configuration.
Understanding the principles of solar panel wiring will enable you to ensure optimal designs for your solar customers. We hope you found this introductory primer helpful!