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.
Key Electrical Terms to Understand for Solar Panel Wiring
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.
Basic Concepts of Solar Panel Wiring (aka Stringing)
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.
Series vs. Parallel Stringing
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.
In this arrangement, 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 parallel 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.
Information You Need When Determining How to String Solar Panels
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.