Free Solar PV String Sizing Tool: FIMER String Sizer

A Web-Based Resource for Optimal Solar Installation Design

Designing a photovoltaic (PV) system requires careful calculation to ensure that the solar panels and inverter are perfectly matched. One of the most critical steps is determining the correct string size — the number of solar panels connected in series to an inverter’s MPPT (Maximum Power Point Tracking) input. An incorrect string size can lead to significant energy losses or even damage to the inverter.

To assist designers, FIMER, a leading global manufacturer of solar inverters, offers a free, web-based tool called the FIMER String Sizer. This article provides an overview of the tool and how it can simplify the design process.

What is the FIMER String Sizer?

The FIMER String Sizer is a complimentary online resource designed to help engineers, installers, and system designers determine the optimum string configuration for a specific PV module and FIMER solar inverter combination. Instead of performing complex manual calculations, users can input a few key parameters, and the tool calculates all allowable string configurations. This provides designers with clear options to choose the most appropriate design for their solar installation.

How the Tool Works

The tool’s process is streamlined to be both powerful and user-friendly.

Step 1 — Specify the design site location: the first step is to define the project’s location, a crucial input as it determines the site’s low and high temperature extremes, which directly affect the voltage and performance of the PV modules.
Step 2 — Select the PV module: the designer selects the specific photovoltaic module to be used in the array. The tool works with a wide range of panels from various manufacturers.
Step 3 — Select the FIMER inverter: the designer chooses the FIMER solar inverter model for the system, and the tool matches the selected PV module with the chosen inverter.
Step 4 — Review allowable configurations: based on the input data, the tool calculates all viable string configurations, determining the number of panels per string and how many strings can be connected in parallel, ensuring the design stays within the operating limits of the inverter.

Essential Calculations Behind the Tool

A tool like the FIMER String Sizer automates vital calculations that are fundamental to safe and efficient PV system design. These calculations, often based on standards like the US National Electrical Code (NEC), are crucial for compliance and performance.

Voltage Calculations

Minimum panels per string: determined by the inverter’s MPPT minimum voltage. At high temperatures, a PV module’s voltage (Vmp) drops, and the string must produce enough voltage to start and operate the inverter’s MPPT tracker.
Maximum panels per string: limited by the inverter’s absolute maximum DC input voltage. At cold temperatures, a PV module’s voltage (Voc) increases, and the total voltage of the string must never exceed this limit or the inverter could be permanently damaged.

Current and Power Calculations

Maximum parallel strings: determined by the inverter’s maximum input current — the combined current from all parallel strings must not exceed the inverter’s rated input current.
Total array power: the tool calculates the total DC power of the array, often expressed in kW DC, helping to optimize the DC/AC ratio for maximum energy harvest.

Who is This Tool For?

Solar system designers, to quickly and accurately evaluate different design scenarios.
Installers and EPCs, to ensure the on-site system is correctly configured for the specific inverter.
Project developers, for preliminary feasibility studies and to optimize system performance.
Students and educators, as a practical tool for learning and teaching solar design principles.

While this tool is specific to FIMER inverters, the underlying principles of string sizing are universal and crucial for all solar PV installations.

Access the Tool

The FIMER String Sizer is a free, web-based application and does not require any software installation.

Disclaimer: this article provides an overview of a third-party tool. While every effort has been made to ensure the information is accurate, always refer to the official FIMER website and the specific product manuals for the most up-to-date and detailed information. Final design responsibility rests with the licensed professional engineer in charge of the project.