How to Design Solar Panel Strings to Best Match Inverters

How many solar panels should each photovoltaic string include? What is the optimal number of photovoltaic strings to connect to an inverter? It’s not as simple as choosing solar panel strings with the same power rating as the inverter.

Due to various factors such as sunlight conditions, installation angles, and line losses, the efficiency of the solar modules cannot achieve 100% output. Most of the time, the output may only be around 70% of the rated power, and even in excellent weather conditions, it can only reach up to 90% of the rated power.

In this article, ADNLITE will share detailed insights on how to design the ratio of solar panel strings to inverters.

Solar Panel Parameters

We have extensively covered the main parameters of solar panels in our Solar Panels Guide. Here, we will still explain some key parameters of solar panel modules.

Standard Test Conditions (STC)

When designing strings, the electrical parameters of the modules are typically chosen under STC conditions. The standard test conditions for solar panels are: Air Mass AM1.5, solar irradiance 1000W/m², and cell operating temperature 25℃.

Temperature Coefficient of Open-Circuit Voltage (Kv)

The temperature coefficient of the open-circuit voltage (Kv) of the module is a negative value. This characteristic means that the open-circuit voltage of the module decreases as the temperature increases and increases as the temperature decreases.

Above are the Electrical Parameters of the solar panel. Let’s remember a few abbreviations:

  • Isc: Short-Circuit Current
  • Im: Peak Operating Current
  • Voc: Open-Circuit Voltage
  • Vm: Peak Operating Voltage

Note that 𝑉𝑚≈0.83×𝑉𝑜𝑐Vm≈0.83×Voc.

Impact of Temperature on Open-Circuit Voltage

The above diagram shows how temperature affects the open-circuit voltage of a module. Now, let’s further explain this effect.

Open-Circuit Voltage Calculation: 𝑉=𝑉𝑜𝑐×[1+(𝑇−25)×𝐾𝑣]V=Voc×[1+(T−25)×Kv]

  • V: Actual open-circuit voltage of the module
  • Voc: Open-circuit voltage of the module under STC conditions
  • T: Ambient temperature
  • Kv: Temperature coefficient of the module

Example:

For the above 420W module with a Voc of 49.2V and a Kv of -0.3%, when the ambient temperature is 0°C, the open-circuit voltage is calculated as:

𝑉=49.2×[1+(0−25)×−0.3%]=52.89𝑉V=49.2×[1+(0−25)×−0.3%]=52.89V

This calculation shows that the open-circuit voltage increases as the temperature decreases.

Impact of Irradiance on Module Voltage and Current

Irradiance and Imp:

  • Within the irradiance range of 100W/m² to 1000W/m², the peak operating current (Imp) increases linearly with the growth of irradiance.

Irradiance and Voc:

  • Under fixed temperature conditions, when the irradiance changes within the range of 400W/m² to 1000W/m², the open-circuit voltage (Voc) remains relatively constant.

Inverter Parameters

We have extensively covered inverter parameters in our The Most Comprehensive Guide To Grid-Tied Inverter Parameters.

Maximum DC Input Voltage:

  • This is the maximum input voltage value for the inverter. It is crucial to consider the actual temperature’s impact on the module’s open-circuit voltage.

Number of MPPT Channels:

  • For strings connected under the same MPPT channel, the number, orientation, and angle of the strings must be consistent.

MPPT Operating Voltage Range:

  • This is the range of Vmppt voltages within which the inverter can operate.

Rated Operating Voltage:

  • The closer the inverter’s operating voltage is to the rated operating voltage, the higher the generation efficiency.

Inverter Efficiency Curve

Operating Voltage and Conversion Efficiency:

  • The conversion efficiency of an inverter is influenced by the operating voltage. Efficiency decreases when the operating voltage is either too low or too high. The closer the operating voltage is to the rated voltage, the higher the conversion efficiency.

Rated Voltage and AC Voltage Relationship:

  • The relationship between the rated voltage and AC voltage is approximately given by: 𝑉≈1.414×𝑉𝑎𝑐+25𝑉V≈1.414×Vac+25V
    • For a 230V grid connection, the rated voltage is generally around 360V.
    • For a 400V grid connection, the rated voltage is generally around 580V.
    • For a 480V grid connection, the rated voltage is generally around 700V.

General Principles for Designing Photovoltaic Strings

The design of solar panel strings needs to satisfy two conditions simultaneously:

  1. The maximum open-circuit voltage of the series-connected photovoltaic modules should be lower than the inverter’s maximum input voltage.
  2. The MPPT voltage of the series-connected photovoltaic modules should fall within the inverter’s MPPT voltage range.

Formula (1) Parameter Definitions:

  • 𝑉𝑑𝑐𝑚𝑎𝑥Vdcmax​: Maximum input voltage of the inverter.
  • The denominator parameters were previously introduced.

Formula (2) Parameter Definitions:

  • 𝑉𝑚𝑝𝑝𝑡𝑚𝑖𝑛Vmpptmin​: Minimum MPPT input voltage of the inverter.
  • 𝑉𝑚𝑝𝑝𝑡𝑚𝑎𝑥Vmpptmax​: Maximum MPPT input voltage of the inverter.
  • 𝑇′T′: Extreme high temperature at the installation site of the modules.
  • 𝑇T: Extreme low temperature at the installation site of the modules.
  • 𝑉𝑝𝑚Vpm​: Peak power voltage of the module.
  • 𝐾𝑣′Kv′: Temperature coefficient of the module’s peak power voltage.
  • 𝑁N: Indicates the range of string numbers that can be connected to the inverter.

By ensuring that the design of the solar panel strings adheres to these principles, the photovoltaic system can operate efficiently and safely within the specified parameters of the inverter.

Example Using MAX 80KTL3 LV Inverter and 420W Modules

Assuming the local extreme low temperature is -25℃ and the extreme high temperature is 50℃. The inverter’s maximum open-circuit voltage is 1100V, and the MPPT voltage range is 200V to 1000V.

Formula (1): 𝑁≤19.44N≤19.44

Formula (2): 5.29≤𝑁≤21.315.29≤N≤21.31

Therefore, the integer value range for 𝑁N is between 6 and 19. Considering the local environmental temperature conditions, the inverter can connect 6 to 19 modules per string.

How to Determine the Optimal Number of Solar Panels for the Inverter:

Principle: The closer the inverter’s Vmppt voltage is to the rated operating voltage, the higher the efficiency and the better the power generation yield. Therefore, the optimal number of series-connected modules should be determined based on this principle.

  1. Calculate the Inverter’s Rated Operating Voltage:
    • For this example, we do not have a specific rated voltage given, so we will use the MPPT range to find an optimal midpoint. Typically, inverters operate most efficiently near the midpoint of their MPPT range.
  2. Midpoint of the MPPT Range:
    • MPPT Range: 200V to 1000V
    • Midpoint: 200𝑉+1000𝑉2=600𝑉2200V+1000V​=600V
  3. Determine the Optimal Number of Modules:
    • Given that the optimal operating voltage is around 600V, we use the module’s peak power voltage (Vmp) to determine the number of modules needed to reach this voltage.
    • For a 420W module, Vmp is typically around 35-38V.
  4. Calculate the Number of Modules:
    • Optimal Number of Modules 𝑁𝑜𝑝𝑡Nopt​: 600𝑉𝑉𝑚𝑝≈600𝑉37𝑉≈16.22Vmp600V​≈37V600V​≈16.22

Since we cannot have a fraction of a module, we round to the nearest whole number. Thus, the optimal number of modules per string is 16.

Call to Action

Unlock the full potential of your solar power system! By leveraging the rated operating voltage parameters provided by inverter manufacturers, you can effortlessly determine the optimal number of modules per string. With inverters boasting a 1.1x overload capacity, your solar panel strings can be configured to deliver peak performance at 1.1 times the rated output.

Don’t settle for less—maximize your energy efficiency and returns! Reach out to ADNLITE. We proudly represent top-tier solar panel and inverter brands (Growatt), and we are committed to providing you with a free, customized solar power system proposal. Join the ranks of satisfied customers who trust ADNLITE to power their future sustainably and efficiently. Contact us today and take the first step towards a brighter, greener tomorrow!