Solar String Sizing

Optimize series and parallel panel configuration.

Panel Specs (STC)

Open Circuit Voltage (Voc)

Short Circuit Current (Isc)

Voc Temp Coeff (%/°C)

Environment

Min Temperature (°C)

Max Temperature (°C)

Inverter Specs

Max Input Voltage (V)

Min MPPT Voltage (V)

Max Panels in Series --

Min Panels in Series --

Max String Voltage -- V

What is Solar String Sizing?

Solar string sizing is the process of determining how many photovoltaic (PV) panels can be connected in series to a single inverter input. This is the most critical step in designing a solar array.

If you connect too many panels, the voltage on a cold winter morning could exceed the inverter's maximum input rating, instantly destroying the electronics. If you connect too few, the voltage on a hot summer day might drop below the inverter's minimum operating range (MPPT window), causing the system to shut down when you need it most.

How to Calculate Max String Size (The Math)

Key to this calculation is the Temperature Coefficient of Voc. Solar ratings are given at Standard Test Conditions (STC, 25°C), but V_oc (Open Circuit Voltage) rises significantly as temperature drops.

1. Calculate Max Voltage (Coldest Day)

This determines the maximum safe string length.

V_{oc_max} = V_{oc_stc} × [1 + (Temp_min - 25) × (Coeff / 100)]

Example: A panel with Voc 40V and coeff -0.3%/°C at -10°C:
40V × [1 + (-10 - 25) × (-0.003)] = 40V × [1 + 0.105] = 44.2V per panel.

If inverter max is 500V: floor(500 / 44.2) = 11 panels max.

2. Calculate Min Voltage (Hottest Day)

This ensures the string voltage stays above the inverter's turn-on threshold.

V_{mp_min} = V_{mp_stc} × [1 + (Temp_max - 25) × (Coeff / 100)]

Practical Applications

Correct string sizing is required for all solar installations:

Grid-Tie Inverters: Most residential string inverters (e.g., SMA, Fronius) have high voltage limits (600V in US, 1000V in EU), allowing long series strings.
Off-Grid Charge Controllers: MPPT controllers (e.g., Victron, Outback) have strict limits like 150V or 250V. Exceeding this is the #1 cause of controller failure.

FAQ

Series vs Parallel: Which is better?

Series increases voltage, keeping current low. This reduces wire size requirements and voltage drop over long distances, making it efficient. Parallel increases current but keeps voltage low; it is better if panels are partially shaded, as one shaded panel won't drag down the entire array.

Does cloud cover affect voltage?

Surprisingly, Voltage remains relatively constant even in low light. It is the Current (Amps) that drops linearly with sunlight intensity. This is why we size for voltage based on temperature, not cloud cover.

What is the "Temperature Coefficient"?

This value (found on the datasheet) tells you how much the voltage changes for every degree Celsius change. It is usually negative (e.g., -0.3%/°C), meaning voltage goes UP as it gets COLDER.