Solar String Sizing for Installers

Reviewed by Max Harris, Quality Assurance Electrician for GreenLancer

Solar string sizing is fundamental to making sure everything in a system runs smoothly. When done right, it helps the photovoltaic (PV) panels and inverters work together efficiently, maximizing solar energy output while also protecting equipment from unnecessary wear and tear. It’s all about getting the details right for a reliable, long-lasting solar power system.

The Risks of Improper Solar String Sizing

When solar string size is incorrect, the consequences can be significant:

• Solar Inverter Damage: Exceeding the voltage or current limits of the inverter can lead to overheating, reduced lifespan, or even equipment failure.

• Efficiency Loss: A poorly sized solar string can prevent your inverter from operating within its ideal voltage range, reducing renewable energy production.

• Safety Hazards: Overvoltage scenarios can create electrical hazards, putting both equipment and installers at risk.

The Role of PV String Sizing in System Design

Accurate solar string sizing is at the core of PV system design. By calculating the correct number of panels per string and aligning it with the inverter’s specifications, you can:

• Maximize renewable energy production

• Maintain system safety under various environmental conditions, including temperature fluctuations

• Ensure compliance with local electrical codes and solar standards.

At GreenLancer, we’ve spent over a decade helping solar installers with essential solar design tasks, including permit-ready solar plan sets and engineering stamps. With our expertise, you can deliver efficient, reliable systems that meet customer expectations and reduce the risk of costly mistakes. 

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What is Solar String Sizing?

Solar string sizing is the process of determining the number of solar panels that can be connected in series within a photovoltaic (PV) system. Each "string" consists of a group of solar panels wired together, and its size is defined by how many panels are included in that string. Solar string size is critical because it directly influences the system's voltage and current output. 

A properly sized string ensures compatibility with the inverter, which is the heart of any solar energy system, as it converts DC power from the panels to AC power for use in homes or businesses. Accurate string sizing also impacts the safety and performance of the system by ensuring that the electrical parameters stay within acceptable ranges under all operating conditions.

How Solar String Size Impacts Voltage, Current, and System Compatibility

• Voltage: The total voltage of a string is determined by adding the open-circuit voltage (Voc) of each panel. This must remain within the inverter's maximum and minimum voltage input range to ensure efficient operation and avoid damage.

• Current: String current is generally determined by the short-circuit current (Isc) of the individual panels. Mismatched current can lead to overheating or inefficiencies in the system.

• System Compatibility: Proper solar string sizing ensures that the panels and inverter work seamlessly, optimizing energy production while maintaining compliance with electrical codes.

Key Terms for Solar String Sizing

Solar String Size:

The number of solar panels connected in series to form a single string. This determines the combined voltage of the string, which must match the inverter's input range.

Voc (Open Circuit Voltage):

The maximum voltage a solar panel produces under standard test conditions with no load connected. It is used to calculate the maximum possible string voltage, especially in cold conditions. This value is listed on the solar panel’s spec sheet and is crucial for calculating string voltage.

Temperature Coefficient of Voltage

A percentage that indicates how much a solar panel’s voltage changes with temperature. A negative coefficient means voltage decreases with higher temperatures and increases with colder temperatures. This value can be found on the PV panel’s datasheet and is essential for factoring in environmental conditions during sizing.

Maximum Solar String Size

The highest number of panels that can be safely connected in series without exceeding the inverter's maximum voltage rating, even under cold weather conditions when panel voltage is highest.

PV String Size

Another term for solar string size, emphasizes its use in photovoltaic (PV) systems.

Solar Panel String Calculation

The process of determining how many panels can be connected in series. This involves using the panel's Voc, temperature coefficient, and the expected temperature extremes for the installation location.

Maximum Solar String Size

The highest number of panels that can be connected in series without exceeding the inverter’s maximum input voltage.

Maximum String Voltage = Voc x (1+ [Temperature Coefficient] × Temperature Delta)

Use the solar panel’s Voc and temperature coefficient, available on the spec sheet. Ensure the calculated voltage stays within the inverter's allowable range at both hot and cold temperature extremes.

Minimum Solar String Size

The lowest number of panels in a string is required to maintain sufficient voltage for the inverter to operate, even on the hottest days when the voltage is lowest.

Key Factors Impacting How to Calculate Solar String Sizing

Solar string sizing involves multiple considerations to balance performance, safety, and compliance. The following factors play a significant role in calculating the correct string size:

Voltage and Current Requirements

To achieve optimal performance, the string voltage must match the inverter’s input voltage range:

• Matching String Voltage to the Inverter: Each inverter is designed to operate within a specific voltage range. If the string voltage exceeds the inverter's maximum input, it can result in overheating or permanent damage. Conversely, a string voltage below the minimum threshold can lead to inefficient energy conversion. Refer to the inverter’s spec sheet to determine its limits.

• Avoiding Overloading with Excessive Current: While string voltage depends on the number of panels in a series, the current is determined by the panel's output. Connecting multiple strings in parallel increases the total current, which must stay within the inverter's allowable input range to avoid damage.

Environmental Conditions & Solar String Size

Temperature variations have a direct impact on the voltage produced by solar panels:

Effects of Temperature on Voltage

• High Temperatures: As temperatures rise, the voltage of each PV panel decreases due to the negative temperature coefficient of voltage.

• Cold Temperatures: In cold weather, the voltage increases. If not accounted for, this can exceed the inverter's maximum voltage limit, potentially causing system failures.

Real-World Examples of PV String Size 

Solar string size must be carefully calculated to accommodate the effects of temperature extremes:

• String Sizing in Hot Climates:

  • In regions like Arizona or Texas, where summer temperatures frequently exceed 100°F (38°C), solar installers often design systems with shorter solar string sizes to prevent the voltage from dropping below the inverter's operational threshold. This ensures the system remains efficient even during the hottest parts of the year.

    
    

• String Sizing in Cold Climates:

  • In places like Minnesota, Alaska, or Canada, winter temperatures can plunge below freezing. Installers might use longer solar string sizes to maximize voltage production during cold conditions. By carefully accounting for the increased voltage in freezing temperatures, you can avoid surpassing the inverter's maximum voltage input, which could otherwise lead to equipment damage.

Maximum PV String Size

Maximum string size is the upper limit on the number of panels you can connect in series to avoid exceeding voltage or current limits.

• The maximum string size defines the number of panels that you can connect without causing the voltage to exceed the inverter’s maximum input voltage. Exceeding this limit could result in inverter damage or PV system failure. It is crucial to calculate the string size carefully to stay within these limits, especially considering temperature variations that may affect panel voltages.

• To determine the maximum string size, you'll need to consider the Voc of the panels, the inverter's voltage range, and environmental factors such as temperature. 

For example, in cold weather, the Voc of the panels will be higher, so you might need to reduce the number of panels per string to stay within the inverter's input range. Tools like the String Length Calculator from PVTools can help you easily calculate the ideal string length based on solar panel specifications and site temperature conditions.

How to Calculate Solar String Size 

Calculating the proper solar string size involves several steps to ensure that the system is both safe and efficient. You can find some of these data points on the spec sheets for the inverter and solar panels. 

Step-by-Step Guide to Solar Panel String Calculation

Here’s a step-by-step guide to help you calculate the solar string size for your PV system:

1. Identify Solar Panel Specifications

Refer to the solar panel spec sheet for key data, including key parameters like Voc, Vmp, and Isc.

• Voc (Open-Circuit Voltage): The maximum voltage the panel will produce under open-circuit conditions. This is a key parameter to ensure that your string voltage does not exceed the inverter's input limits.

• Vmp (Maximum Power Voltage): The voltage at which the panel produces its maximum power. This value is useful for understanding the operational voltage but isn’t as critical for string sizing as Voc.

• Isc (Short-Circuit Current): The maximum current the panel can produce under short-circuit conditions. This value is important to ensure that the current from your string does not exceed the inverter's current rating.

2. Use the Inverter’s Operating Voltage Range to Determine String Limits

Every PV inverter has an operating voltage range, which typically includes a minimum and maximum voltage.

• Calculate the string voltage by multiplying the panel's Voc by the number of panels you plan to connect in series. Ensure that the maximum voltage of the string stays within the inverter’s maximum voltage input range.

• For example, if the inverter's maximum voltage is 600V and each panel has a Voc of 40V, you can safely connect up to 15 panels in series (40V * 15 = 600V).

3. Account for Temperature Adjustment Factors

Temperature can significantly affect the Voc of solar panels, especially in very hot or cold climates.

• Temperature Coefficient: The temperature coefficient for Voc indicates how much the voltage will change with temperature. For example, if the temperature coefficient is -0.3%/°C, and the ambient temperature drops by 10°C, the Voc of the panel will increase by 3% (0.3% * 10).

• Always adjust the string calculation based on expected temperature variations to ensure the system does not exceed the inverter’s voltage rating. You can find this information on the spec sheet for the chosen solar inverter.

  
  

Tools for Solar String Size Calculations

These tools ensure accuracy and streamline the solar string sizing process, helping you avoid costly errors and ensure maximum efficiency.  It's crucial to verify that your string sizing is compatible with the inverter, so refer to the inverter’s spec sheet to confirm compatibility. 

There are several software and tools available to make solar string sizing easier:

• Recommended Software and Calculators:

  • PVsyst: A comprehensive tool for solar system design, including string sizing.

  • SAM (System Advisor Model): A modeling tool from NREL that helps with designing and sizing solar power systems.

    
    

• Online Resources and Manufacturer Tools:

  • PVTools String Length Calculator: This free online tool lets you calculate the maximum number of panels per string based on temperature, panel specs, and inverter limits.

  • Inverter Manufacturer Tools: Most inverter manufacturers, like SMA and Fronius, offer string sizing calculators on their websites to help installers ensure compatibility between panels and inverters.

3 Common Mistakes in Solar String Sizing & How to Avoid Them

Here are some of the common mistakes we have seen here at GreenLancer over the last decade. 

Overlooking Temperature Effects on Voltage

One of the most common mistakes when calculating solar string size is neglecting the impact of temperature on panel voltage. As temperature decreases, the Voc increases, and if this isn’t factored into your calculations, it can push the system’s voltage beyond the inverter’s limits. Always consider the temperature coefficient and adjust your calculations based on expected environmental conditions.

Mismatching Inverter and Panel Specs

Another common mistake with solar string sizing is selecting panels that are incompatible with the inverter’s voltage and current ratings. Ensure that the panels' Voc and Isc fall within the inverter's range. Also, the combined current of the panels should not exceed the inverter's maximum current limit. Check both voltage and current values to confirm compatibility before finalizing the string size.

Missing Maximum String Size Limits

Many installers overlook the importance of the maximum solar string size, which determines the upper limit for the number of solar modules you can string together. Exceeding this limit can lead to solar PV system inefficiencies or even damage the inverter. Make sure to calculate the maximum number of solar panels that can be safely connected and never exceed the inverter’s maximum input voltage or current limits.

Real-World Examples of PV String Size Calculations

Let’s examine some hypothetical examples to learn about how to calculate solar string size.

Residential Solar String Size Calculation

• Assume each panel has a Voc of 40V, and you are using an inverter with a maximum input voltage of 600V.

• If the expected temperature is 25°C (with a temperature coefficient of -0.3%/°C), the Voc at that temperature will remain the same.

• You can safely string up to 15 panels in series (40V * 15 = 600V) without exceeding the inverter’s voltage limit.

• The inverter has a maximum current rating of 10A, and the panel Isc is 9A, so this string is well within the current limit.

Commercial Solar String Size Calculation

Now, let’s look at a larger commercial or utility-scale system.

• Assume each panel has a Voc of 40V, and the inverter’s maximum voltage is 1000V.

• In a colder environment, where the temperature could drop to 0°C, the Voc might increase by 3%, so the adjusted Voc per panel would be 41.2V (40V * 1.03).

• The inverter can handle up to 24 panels in series (1000V ÷ 41.2V = 24.2, rounded down to 24 panels).

• If the total current is within the inverter's current rating, this string will work efficiently without overloading the system.

Optimizing Solar String Sizing 

Proper solar string sizing is a critical step in designing a safe, efficient, and durable solar PV system. By accounting for factors such as environmental conditions, panel specifications, and inverter requirements, you can ensure optimal performance while avoiding potential system issues.

If you're looking for expert support in solar design, engineering, or interconnection services, GreenLancer is here to help. Our experienced team simplifies the process, providing fast, reliable, and cost-effective solutions tailored to your project's needs. Whether you need assistance with PV string sizing or full system design, GreenLancer ensures your project moves seamlessly from concept to commissioning.