Solar PV String Sizing & Inverter Matching Calculator

Module Open Circuit Voltage (Voc, V):

Module Max Power Voltage (Vmp, V):

Temperature Coefficient (%/°C):

Lowest Ambient Temperature (°C):

Highest Ambient Temperature (°C):

Inverter Max DC Voltage (V):

Inverter Min MPPT Voltage (V):

Inverter Max MPPT Voltage (V):

How to Use the Solar PV String Sizing & Inverter Matching Calculator

A Solar PV String Sizing & Inverter Matching Calculator is a tool that helps solar designers and installers determine how many photovoltaic (PV) modules can be connected in series per string while ensuring compatibility with the inverter’s voltage limits under varying temperature conditions.

Why Is This Important?

When designing solar PV systems, it’s critical to match the string voltage to the inverter’s maximum DC input voltage and MPPT operating range. Temperature fluctuations significantly affect solar module voltages:

Cold weather increases voltage (risking inverter overvoltage damage).
Hot weather decreases voltage (risking inverter dropout if voltage falls below MPPT minimum).

If string sizing isn’t done correctly, the system could:

Fail to operate at certain temperatures.
Reduce efficiency.
Damage the inverter.

This calculator automates the process, making the design safer and faster.

How to Use the Calculator

Enter Module Data
- Voc (Open Circuit Voltage): The maximum voltage of the panel under no load.
- Vmp (Max Power Voltage): The voltage at which the panel produces maximum power.
- Temperature Coefficient (%/°C): Usually a negative value provided in the datasheet.
Enter Site Conditions
- Lowest Ambient Temperature (°C): Coldest expected site temperature (worst case for high voltage).
- Highest Ambient Temperature (°C): Hottest expected site temperature (worst case for low voltage).
Enter Inverter Specifications
- Inverter Max DC Voltage (V): The maximum allowable DC input voltage.
- Inverter Min MPPT Voltage (V): The lowest voltage the inverter MPPT can track.
- Inverter Max MPPT Voltage (V): The upper MPPT voltage limit for tracking efficiency.
Click “Calculate String Size”
- You’ll see:
  - Max Modules in Series (to avoid exceeding the inverter’s max voltage).
  - Min Modules in Series (to stay above inverter min voltage at high temps).
  - Recommended String Range (safe operational range considering MPPT).

Example

Suppose you have:

Voc = 40V, Vmp = 33V, Temp Coefficient = -0.3%/°C
Site Temp Range: -5°C to 45°C
Inverter Specs: Max DC = 1000V, MPPT range = 300–800V

The calculator might return:

Max Modules in Series = 22
Min Modules in Series = 10
Recommended Range = 10 – 24 modules per string

This ensures that in the coldest weather, you won’t overvoltage the inverter, and in the hottest weather, the system stays above MPPT minimum.

FAQ – Solar PV String Sizing & Inverter Matching Calculator

Q1: Why do we correct voltages for temperature?
A: PV module voltages vary with temperature. Cold increases voltage, hot decreases voltage. If you don’t account for this, the inverter may shut down or be damaged.

Q2: Why does the calculator suggest a range instead of a fixed number?
A: Because both minimum and maximum limits matter. The range ensures strings operate properly across seasonal extremes.

Q3: What happens if I oversize the string beyond max voltage?
A: The inverter could be permanently damaged due to overvoltage stress. Always stay below the manufacturer’s stated max DC input.

Q4: What if I undersize the string below the MPPT minimum?
A: The inverter will not start tracking power, reducing energy yield significantly.

Q5: Does this calculator consider current (Isc) or power limits?
A: No, this version focuses on voltage-based string sizing. Designers should also check inverter current limits separately.

Q6: Can this tool be used for off-grid solar systems?
A: Yes, but it is mainly designed for grid-tied systems where string sizing relative to inverter specs is most critical.