Solar String Sizing Calculator (Modules per String, Voc & Vmp)

How to use the solar string sizing calculator

Enter the module Voc and Vmp at STC and their temperature coefficients from the datasheet, the site’s lowest ambient temperature and the highest cell temperature, then the inverter’s maximum DC voltage and MPPT range. The calculator gives the temperature-corrected module voltages, the maximum and minimum modules per string, and the string voltages for the length you enter in Modules per string.

Solar string sizing formulas

V_oc,cold = V_oc × [1 + β_Voc × (T_min − 25 °C)]  ·  N_max = ⌊V_max,inv ÷ V_oc,cold⌋

V_mp,hot = V_mp × [1 + β_Vmp × (T_cell,max − 25 °C)]  ·  N_min = ⌈V_MPPT,min ÷ V_mp,hot⌉

A string length is acceptable when N_min ≤ N ≤ N_max. For best energy yield, also keep N ≤ the MPPT-range limit so the cold-weather Vmp stays inside the tracking window.

Worked example

A 49.5 V Voc / 41.6 V Vmp module (β_Voc = -0.27 %/°C, β_Vmp = -0.35 %/°C), site low 0 °C, hot cell temperature 70 °C, inverter 1,000 V max, MPPT 200–850 V:

• V_oc,cold = 49.5 × (1 + 0.0027 × 25) = 52.84 V → N_max = ⌊1000 ÷ 52.84⌋ = 18
• V_mp,hot = 41.6 × (1 − 0.0035 × 45) = 35.05 V → N_min = ⌈200 ÷ 35.05⌉ = 6
• Cold Vmp = 45.24 V → at most 18 modules stay inside the 850 V MPPT limit
• A 16-module string: Voc on the coldest morning = 845.5 V, Vmp on the hottest afternoon = 560.8 V — both within limits

Cold-weather Voc and maximum string length (49.5 V module, -0.27 %/°C)

Practical notes

• Check current too: the string Isc × 1.25 (and parallel strings per MPPT) must not exceed the inverter’s maximum input current.
• Use the inverter’s start-up voltage if it is higher than the MPPT minimum.
• Bifacial and high-efficiency modules may have different coefficients — always take β from the exact datasheet.
• Shading, soiling and mismatch lower Vmp further; leave a margin above the MPPT minimum rather than designing exactly to it.

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