String Sizing

What is string sizing

String sizing is the design process of determining the number of solar modules connected in series to form a string that feeds one MPPT input of a solar inverter. The optimal string length is constrained by the inverter’s MPPT voltage window and the temperature-dependent behaviour of module open-circuit voltage (Voc) and voltage at maximum power (Vmp).

The two binding constraints:

1. Maximum string Voc at coldest expected temperature must stay below the inverter’s absolute maximum DC input voltage.
2. Minimum string Vmp at hottest expected operating temperature must stay above the inverter’s MPPT lower limit.

A string longer than the upper limit can damage the inverter on the first cold morning. A string shorter than the lower limit forces the inverter outside its MPPT window during hot periods, losing yield. See string voltage vs MPPT voltage in solar inverters for the full temperature-corrected design framework Indian EPCs use across cold Himalayan mornings and 65°C+ Rajasthan rooftops.

Why string sizing matters

Inverter safety. Exceeding maximum DC input destroys the inverter.

Yield optimisation. String voltage within MPPT window maximises power tracking.

Equipment compatibility. Verified string sizing makes inverter, modules and cabling work as a system.

ALMM compliance. ALMM and BIS certificates assume sized strings.

Regulatory acceptance. CEIG and DISCOM commissioning verify string configuration.

String sizing equations

Upper bound (cold morning)

Where:

• N = number of modules in series
• βVoc = temperature coefficient of Voc (negative number, in %/°C as decimal)
• Tmin_cell = minimum expected cell temperature (≈ ambient at dawn)

Constraint: Voc_string_cold ≤ V_inverter_max_DC

Lower bound (hot noon)

Constraint: Vmp_string_hot ≥ V_MPPT_lower

Technical deep dive

Worked example (Indian C&I rooftop)

Indian regional Tmin assumptions

Temperature coefficient by cell technology

Common Indian residential string ranges

Real-world applications

Residential rooftop solar design.

C&I rooftop and ground-mount string layout.

Utility solar plant configuration.

Carport string design.

Floating solar with seasonal temperature variation.

Common string sizing problems

String too long, inverter destroyed on cold morning.

String too short, inverter de-rates at noon during summer.

Mixed module brands creating Vmp mismatch.

Cold-temperature derate ignored for Ladakh-class sites.

Different orientations on same string causing mismatch.

Best practices

Use site-specific Tmin and Tmax for derating.

Apply 5 percent safety margin on maximum Voc.

Document string sizing on the single-line diagram, including the fuse rating for parallel strings routed through a combiner box; see solar string sizing and overcurrent protection in India for the 5-step method EPC teams use to size both together.

Verify with PVsyst, the inverter manufacturer’s sizing tool, or a browser-based solar design tool.

Match string lengths within one MPPT.

Use one MPPT per orientation.

Specify cold-climate ratings for high-altitude installations.

Comparison: residential vs utility string sizing

Indian market context

Inverter manufacturers (Sungrow, Solis, Goodwe, Solax, Growatt, Deye) publish online string sizing tools.

PVsyst is the industry standard for utility-scale string sizing in India.

ALMM-listed module datasheets provide Voc, Vmp and temperature coefficients required for sizing.

CEIG drawing approval includes verification of string configuration.

PM Surya Ghar residential installations use installer-side sizing tools.

Standards and certifications

Common mistakes

Using STC values without temperature derate.

Ignoring cold-temperature Voc rise on high-altitude sites.

Mixing 60-cell and 72-cell modules in one string.

Skipping inverter MPPT window check.

Using same string size across different orientations.

Not documenting string sizing assumptions.

Trusting installer recommendation without verification.

Key takeaways

String sizing is the design step that matches modules to inverter MPPT window across temperatures. Maximum string Voc at coldest temperature must stay below inverter max DC input. Minimum string Vmp at hottest temperature must stay above MPPT lower limit. Site-specific temperature assumptions, cell-technology coefficients and inverter datasheet limits determine acceptable string lengths. Wrong string sizing destroys inverters or wastes yield.

Further reading

For how String Sizing plays out in real projects, these guides go deeper:

• Solar String Sizing Software: Complete 2026 Guide
• Solar String Sizing & Overcurrent Protection
• String Voltage vs MPPT Voltage: Inverter Guide
• Solar Inverter Sizing: 12 Key Questions (2026)

Sources

• Manufacturer string sizing tools from Sungrow, Solis, Goodwe, Solax, Growatt, Deye.
• PVsyst software documentation.
• IS 14286, Indian Standard for crystalline silicon PV modules.
• CEA (Technical Standards for Connectivity of the Distributed Generation Resources) Regulations, 2013 (as amended 2019), Central Electricity Authority.
• MNRE Design Guidelines for Solar PV Plants.
• IEC 61730, Module safety standards.
• Inverter datasheets from major OEMs.
• IEA PVPS Trends in Photovoltaic Applications 2024.
• Duffie J. A., Beckman W. A., Solar Engineering of Thermal Processes, Wiley.